Andi,
You _again_ 'forgot' to Cc: peterz who is an affected maintainer and who
is keenly interested in such low level changes affecting scheduling - and
he asked to be Cc:-ed on your previous submission.
I still don't understand, why do you *routinely* do office politics crap
like that, playing games with Cc:s and private mails, which eminently
hinders kernel development? (Oh, it's deniable and I'm quite sure you'll
deny it in a heartbeat and call it an inadvertent omission. Just skip the
excuses and stop it, ok?)
Thanks,
Ingo
* Andi Kleen <[email protected]> wrote:
> The x86 user access functions (*_user) were originally very well tuned,
> with partial inline code and other optimizations.
>
> Then over time various new checks -- particularly the sleep checks for
> a voluntary preempt kernel -- destroyed a lot of the tunings
>
> A typical user access operation is now doing multiple useless
> function calls. Also the without force inline gcc's inlining
> policy makes it even worse, with adding more unnecessary calls.
>
> Here's a typical example from ftrace:
>
> 10) | might_fault() {
> 10) | _cond_resched() {
> 10) | should_resched() {
> 10) | need_resched() {
> 10) 0.063 us | test_ti_thread_flag();
> 10) 0.643 us | }
> 10) 1.238 us | }
> 10) 1.845 us | }
> 10) 2.438 us | }
>
> So we spent 2.5us doing nothing (ok it's a bit less without
> ftrace, but still pretty bad)
>
> Then in other cases we would have an out of line function,
> but would actually do the might_sleep() checks in the inlined
> caller. This doesn't make any sense at all.
>
> There were also a few other problems, for example the x86-64 uaccess
> code regularly falls back to string functions, even though a simple
> mov would be enough. For example every futex access to the lock
> variable would actually use string instructions, even though
> it's just 4 bytes.
>
> This patch kit is an attempt to get us back to sane code,
> mostly by doing proper inlining and doing sleep checks in the right
> place. Unfortunately I had to add one tree sweep to avoid an nasty
> include loop.
>
> v2: Now completely remove reschedule checks for uaccess functions.
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