RE: userret() , ast() and the end of syscalls
On Wed, 10 Jul 2002, John Baldwin wrote: > On 09-Jul-2002 Julian Elischer wrote: > > On Wed, 10 Jul 2002, Bruce Evans wrote: > >> Can these flags be changed asynchronously? If so, then everything needs > >> to be handled by ast() anyway. userret() should only check for work that > >> needs doing in the usual case, and hopefully there is none (except for > >> things like ktrace). > > > > That's an interestign way of thinking about it.. I think of ast() as the real userret(). Splitting them is an implementation detail. The real userret() has to be very close to the return to user mode, since the return needs to be atomic with checking for things that need to be done before return, so you need fairly strong locking and don't want to hold the lock for very long. > > in that case, shouldn't ast() be called from within userret() > > instead of the other way around? No; that would pessimize userret() and break ast(). ast() is the "real" userret() so i needs to do more. > > userret() is called unconditionally from both trap() and syscall() > > (or just trap() on architectures where syscall() is called by trap()) > > > > if teh tast thing userret() did was to check if ast() should be called > > and to call it, it might simplify things.. > > also, should userret() then loop back to it's start if trap is called? > > It would need to, to simulate what it is doing now.. > > The test you refer to is done in MD code because ensuring atomicity involves > doing MD things like disabling interrupts. It really works quite well the > way it is atm. John moved the loop into ast() where it is easier to understand, but Matt made him move it back. There would be little point in userret() repeating the loop, since it "has" to open up a race window on return and then it would miss state changes. These should be seen later by ast(). Bruce To Unsubscribe: send mail to [EMAIL PROTECTED] with "unsubscribe freebsd-current" in the body of the message
RE: userret() , ast() and the end of syscalls
On 09-Jul-2002 Julian Elischer wrote: > > > On Wed, 10 Jul 2002, Bruce Evans wrote: >> Can these flags be changed asynchronously? If so, then everything needs >> to be handled by ast() anyway. userret() should only check for work that >> needs doing in the usual case, and hopefully there is none (except for >> things like ktrace). > > That's an interestign way of thinking about it.. > in that case, shouldn't ast() be called from within userret() > instead of the other way around? > > userret() is called unconditionally from both trap() and syscall() > (or just trap() on architectures where syscall() is called by trap()) > > > if teh tast thing userret() did was to check if ast() should be called > and to call it, it might simplify things.. > also, should userret() then loop back to it's start if trap is called? > It would need to, to simulate what it is doing now.. The test you refer to is done in MD code because ensuring atomicity involves doing MD things like disabling interrupts. It really works quite well the way it is atm. -- John Baldwin <[EMAIL PROTECTED]> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/ To Unsubscribe: send mail to [EMAIL PROTECTED] with "unsubscribe freebsd-current" in the body of the message
RE: userret() , ast() and the end of syscalls
On Wed, 10 Jul 2002, Bruce Evans wrote: > Can these flags be changed asynchronously? If so, then everything needs > to be handled by ast() anyway. userret() should only check for work that > needs doing in the usual case, and hopefully there is none (except for > things like ktrace). That's an interestign way of thinking about it.. in that case, shouldn't ast() be called from within userret() instead of the other way around? userret() is called unconditionally from both trap() and syscall() (or just trap() on architectures where syscall() is called by trap()) if teh tast thing userret() did was to check if ast() should be called and to call it, it might simplify things.. also, should userret() then loop back to it's start if trap is called? It would need to, to simulate what it is doing now.. now: userret() | | | v doreti() ? <-->ast()<-->userret() | | | v userland maybe: userret() ? <-->ast() (maybe loop back to top of userret) | | | v doreti() | | | v userland One other question? why is userret called again if ast() is called? It seems about the only thing left in it is the profiling flags. (under normal conditions) To Unsubscribe: send mail to [EMAIL PROTECTED] with "unsubscribe freebsd-current" in the body of the message
RE: userret() , ast() and the end of syscalls
On Wed, 10 Jul 2002, Bruce Evans wrote:
> On Tue, 9 Jul 2002, Julian Elischer wrote:
>
> > On Wed, 10 Jul 2002, Bruce Evans wrote:
> >
> > > Hopefully there won't be any unconditional code. Unconditional code
> > > in userret() pessimizes all syscalls. Unconditional code added by KSEIII
> > > pessimized basic syscall overhead by 10% according to lmbench2.
> >
> > Mostly it's conditional..
> > if (p->p_flag & P_KSES)
> > in syscall()
> > and
> > if (p->p_flag & P_KSES) {
> > in userret()
>
> The conditionals are unconditional, and together with the proc locking)
> (mainly the locking) are what gives the 10% pessimization. It would be
> much more than 10% to actually do something :).
>
> > it's probably
> > PROC_LOCK(p);
> > thread_suspend_check(0);/* Can suspend or kill */
> > PROC_UNLOCK(p);
> >
> >
> > try replace it with:
> > if (P_SHOULDSTOP(p) {
> > PROC_LOCK(p);
> > thread_suspend_check(0);/* Can suspend or kill */
> > PROC_UNLOCK(p);
> > }
>
> Can these flags be changed asynchronously? If so, then everything needs
> to be handled by ast() anyway. userret() should only check for work that
> needs doing in the usual case, and hopefully there is none (except for
> things like ktrace).
That (use ast) is a sensible suggestion. I guess it belongs
with the postsig in ast().
I can try that.. though it ;ll take a little work
in the mean time try the patch above with lmbench..
>
> Bruce
>
>
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RE: userret() , ast() and the end of syscalls
On Tue, 9 Jul 2002, Julian Elischer wrote:
> On Wed, 10 Jul 2002, Bruce Evans wrote:
>
> > Hopefully there won't be any unconditional code. Unconditional code
> > in userret() pessimizes all syscalls. Unconditional code added by KSEIII
> > pessimized basic syscall overhead by 10% according to lmbench2.
>
> Mostly it's conditional..
> if (p->p_flag & P_KSES)
> in syscall()
> and
> if (p->p_flag & P_KSES) {
> in userret()
The conditionals are unconditional, and together with the proc locking)
(mainly the locking) are what gives the 10% pessimization. It would be
much more than 10% to actually do something :).
> it's probably
> PROC_LOCK(p);
> thread_suspend_check(0);/* Can suspend or kill */
> PROC_UNLOCK(p);
>
>
> try replace it with:
> if (P_SHOULDSTOP(p) {
> PROC_LOCK(p);
> thread_suspend_check(0);/* Can suspend or kill */
> PROC_UNLOCK(p);
> }
Can these flags be changed asynchronously? If so, then everything needs
to be handled by ast() anyway. userret() should only check for work that
needs doing in the usual case, and hopefully there is none (except for
things like ktrace).
Bruce
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RE: userret() , ast() and the end of syscalls
On Wed, 10 Jul 2002, Bruce Evans wrote:
> Hopefully there won't be any unconditional code. Unconditional code
> in userret() pessimizes all syscalls. Unconditional code added by KSEIII
> pessimized basic syscall overhead by 10% according to lmbench2.
Mostly it's conditional..
if (p->p_flag & P_KSES)
in syscall()
and
if (p->p_flag & P_KSES) {
in userret()
it's probably
PROC_LOCK(p);
thread_suspend_check(0);/* Can suspend or kill */
PROC_UNLOCK(p);
try replace it with:
if (P_SHOULDSTOP(p) {
PROC_LOCK(p);
thread_suspend_check(0);/* Can suspend or kill */
PROC_UNLOCK(p);
}
>
> Bruce
>
>
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RE: userret() , ast() and the end of syscalls
On Tue, 9 Jul 2002, John Baldwin wrote: > On 09-Jul-2002 John Baldwin wrote: > > > > On 09-Jul-2002 Julian Elischer wrote: > >> > >> A question to those who know.. > >> > >> why is userret() called both at the end of trap() or syscall() > >> and also almost immediatly again (often) at the end of ast(). > > > > ast() is really a special form of a trap that is triggered by doing > > a last-minute type check on return to userland to see if we still > > have work to do. > > > >> It seems that really there is no one place that one can put code that will > >> be called ONCE and ONLY ONCE as a thread progresses to userland. > > > > Sure there is. When you want an action done, set a thread flag marking > > the request and set TDF_ASTPENDING. Then handle it in ast() if the flag > > is set. > > Or, if this needs to happen on every return and not conditionally, > then do it in userret() and use the state of a variable or some flag > to note when you've already done it. Hopefully there won't be any unconditional code. Unconditional code in userret() pessimizes all syscalls. Unconditional code added by KSEIII pessimized basic syscall overhead by 10% according to lmbench2. Bruce To Unsubscribe: send mail to [EMAIL PROTECTED] with "unsubscribe freebsd-current" in the body of the message
Re: userret() , ast() and the end of syscalls
- Original Message - From: "John Baldwin" <[EMAIL PROTECTED]> To: "John Baldwin" <[EMAIL PROTECTED]> Cc: "FreeBSD current users" <[EMAIL PROTECTED]>; "FreeBSD current users" <[EMAIL PROTECTED]>; "Julian Elischer" <[EMAIL PROTECTED]> Sent: Tuesday, July 09, 2002 8:40 PM Subject: RE: userret() , ast() and the end of syscalls > > On 09-Jul-2002 John Baldwin wrote: > > > > On 09-Jul-2002 Julian Elischer wrote: > >> > >> A question to those who know.. > >> > >> why is userret() called both at the end of trap() or syscall() > >> and also almost immediatly again (often) at the end of ast(). > > > > ast() is really a special form of a trap that is triggered by doing > > a last-minute type check on return to userland to see if we still > > have work to do. > > > >> It seems that really there is no one place that one can put code that will > >> be called ONCE and ONLY ONCE as a thread progresses to userland. > > > > Sure there is. When you want an action done, set a thread flag marking > > the request and set TDF_ASTPENDING. Then handle it in ast() if the flag > > is set. > > Or, if this needs to happen on every return and not conditionally, > then do it in userret() and use the state of a variable or some flag > to note when you've already done it. > > -- > > John Baldwin <[EMAIL PROTECTED]> <>< http://www.FreeBSD.org/~jhb/ > "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/ hope this won't increase per-thread memory requirement, I would like to create more threads using same memory size. :) David Xu __ Do You Yahoo!? Sign up for SBC Yahoo! Dial - First Month Free http://sbc.yahoo.com To Unsubscribe: send mail to [EMAIL PROTECTED] with "unsubscribe freebsd-current" in the body of the message
RE: userret() , ast() and the end of syscalls
On 09-Jul-2002 John Baldwin wrote: > > On 09-Jul-2002 Julian Elischer wrote: >> >> A question to those who know.. >> >> why is userret() called both at the end of trap() or syscall() >> and also almost immediatly again (often) at the end of ast(). > > ast() is really a special form of a trap that is triggered by doing > a last-minute type check on return to userland to see if we still > have work to do. > >> It seems that really there is no one place that one can put code that will >> be called ONCE and ONLY ONCE as a thread progresses to userland. > > Sure there is. When you want an action done, set a thread flag marking > the request and set TDF_ASTPENDING. Then handle it in ast() if the flag > is set. Or, if this needs to happen on every return and not conditionally, then do it in userret() and use the state of a variable or some flag to note when you've already done it. -- John Baldwin <[EMAIL PROTECTED]> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/ To Unsubscribe: send mail to [EMAIL PROTECTED] with "unsubscribe freebsd-current" in the body of the message
Re: userret() , ast() and the end of syscalls
On 09-Jul-2002 David Xu wrote: > I found the problem two weeks ago, but I can not find a better way to > avoid userret() to be called twice. so I keep silence. :( It is only called twice if an AST is posted. It is _not_ always called twice. -- John Baldwin <[EMAIL PROTECTED]> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/ To Unsubscribe: send mail to [EMAIL PROTECTED] with "unsubscribe freebsd-current" in the body of the message
RE: userret() , ast() and the end of syscalls
On 09-Jul-2002 Julian Elischer wrote: > > A question to those who know.. > > why is userret() called both at the end of trap() or syscall() > and also almost immediatly again (often) at the end of ast(). ast() is really a special form of a trap that is triggered by doing a last-minute type check on return to userland to see if we still have work to do. > It seems that really there is no one place that one can put code that will > be called ONCE and ONLY ONCE as a thread progresses to userland. Sure there is. When you want an action done, set a thread flag marking the request and set TDF_ASTPENDING. Then handle it in ast() if the flag is set. > There is no one place where you can say "after this point we are in > userland" right up until that iret instruction. There is always the danger > that FTER ny insruction that decides that we are now definitly going to > user space, there could occur an interrupt that causes a reschedule so > that some OTHER thread goes to user land. > > Is it possible to clear interrupts and have the iret instruction > itself re-enable them? > (that would give a few instructions 'atomically' with the iret > which may be all I need). Remember how ast() used to do a critical_enter() before it grabbed sched_lock to check flags and dropped the critical section while handling the flags? This is exactly what you have asked for above. The loop and the critical sections then got pushed back into the MD code but still work the same. > is this possible in other architectures? It already works that way on all architectures. -- John Baldwin <[EMAIL PROTECTED]> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/ To Unsubscribe: send mail to [EMAIL PROTECTED] with "unsubscribe freebsd-current" in the body of the message
Re: userret() , ast() and the end of syscalls
I found the problem two weeks ago, but I can not find a better way to avoid userret() to be called twice. so I keep silence. :( David Xu Gartner: Apache is vulnerable, we recommend switching back to IIS to protect yourselves - Original Message - From: "Julian Elischer" <[EMAIL PROTECTED]> To: "FreeBSD current users" <[EMAIL PROTECTED]> Sent: Tuesday, July 09, 2002 2:40 PM Subject: userret() , ast() and the end of syscalls > > A question to those who know.. > > why is userret() called both at the end of trap() or syscall() > and also almost immediatly again (often) at the end of ast(). > > It seems that really there is no one place that one can put code that will > be called ONCE and ONLY ONCE as a thread progresses to userland. > > There is no one place where you can say "after this point we are in > userland" right up until that iret instruction. There is always the danger > that FTER ny insruction that decides that we are now definitly going to > user space, there could occur an interrupt that causes a reschedule so > that some OTHER thread goes to user land. > > Is it possible to clear interrupts and have the iret instruction > itself re-enable them? > (that would give a few instructions 'atomically' with the iret > which may be all I need). > > is this possible in other architectures? > > > > To Unsubscribe: send mail to [EMAIL PROTECTED] > with "unsubscribe freebsd-current" in the body of the message To Unsubscribe: send mail to [EMAIL PROTECTED] with "unsubscribe freebsd-current" in the body of the message
