Resending in case people missed the previous message.
On Fri, Aug 1, 2025 at 10:30 AM Wander Lairson Costa <wan...@redhat.com> wrote:
>
> On Tue, Jul 8, 2025 at 3:54 PM Peter Zijlstra <pet...@infradead.org> wrote:
> >
> > On Tue, Jul 08, 2025 at 09:54:06AM -0300, Wander Lairson Costa wrote:
> > > O Mon, Jul 07, 2025 at 01:20:03PM +0200, Peter Zijlstra wrote:
> > > > On Fri, Jul 04, 2025 at 02:07:43PM -0300, Wander Lairson Costa wrote:
> > > > > Similar to the IRQ tracepoint, the preempt tracepoints are typically
> > > > > disabled in production systems due to the significant overhead they
> > > > > introduce even when not in use.
> > > > >
> > > > > The overhead primarily comes from two sources: First, when tracepoints
> > > > > are compiled into the kernel, preempt_count_add() and
> > > > > preempt_count_sub()
> > > > > become external function calls rather than inlined operations. Second,
> > > > > these functions perform unnecessary preempt_count() checks even when
> > > > > the
> > > > > tracepoint itself is disabled.
> > > > >
> > > > > This optimization introduces an early check of the tracepoint static
> > > > > key,
> > > > > which allows us to skip both the function call overhead and the
> > > > > redundant
> > > > > preempt_count() checks when tracing is disabled. The change maintains
> > > > > all
> > > > > existing functionality when tracing is active while significantly
> > > > > reducing overhead for the common case where tracing is inactive.
> > > > >
> > > >
> > > > This one in particular I worry about the code gen impact. There are a
> > > > *LOT* of preempt_{dis,en}able() sites in the kernel and now they all get
> > > > this static branch and call crud on.
> > > >
> > > > We spend significant effort to make preempt_{dis,en}able() as small as
> > > > possible.
> > > >
> > >
> > > Thank you for the feedback, it's much appreciated. I just want to make
> > > sure
> > > I'm on the right track. If I understand your concern correctly, it
> > > revolves
> > > around the overhead this patch might introduce???specifically to the
> > > binary
> > > size and its effect on the iCache???when the kernel is built with preempt
> > > tracepoints enabled. Is that an accurate summary?
> >
> > Yes, specifically:
> >
> > preempt_disable()
> > incl %gs:__preempt_count
> >
> >
> >
> > preempt_enable()
> > decl %gs:__preempt_count
> > jz do_schedule
> > 1: ...
> >
> > do_schedule:
> > call __SCT__preemptible_schedule
> > jmp 1
> >
> >
> > your proposal adds significantly to this.
> >
>
Here is a breakdown of the patch's behavior under the different kernel
configurations:
* When DEBUG_PREEMPT is defined, the behavior is identical to the
current implementation, with calls to preempt_count_add/sub().
* When both DEBUG_PREEMPT and TRACE_PREEMPT_TOGGLE are disabled, the
generated code is also unchanged.
* The primary change occurs when only TRACE_PREEMPT_TOGGLE is defined.
In this case, the code uses a static key test instead of a function
call. As the benchmarks show, this approach is faster when the
tracepoints are disabled.
The main trade-off is that enabling or disabling these tracepoints
will require the kernel to patch more code locations due to the use of
static keys.
