Hello,
On Mon, Nov 23, 2020 at 8:00 PM Michael Ellerman <m...@ellerman.id.au> wrote:
>
> Namhyung Kim <namhy...@kernel.org> writes:
> > Hi Peter and Kan,
> >
> > (Adding PPC folks)
> >
> > On Tue, Nov 17, 2020 at 2:01 PM Namhyung Kim <namhy...@kernel.org> wrote:
> >>
> >> Hello,
> >>
> >> On Thu, Nov 12, 2020 at 4:54 AM Liang, Kan <kan.li...@linux.intel.com>
> >> wrote:
> >> >
> >> >
> >> >
> >> > On 11/11/2020 11:25 AM, Peter Zijlstra wrote:
> >> > > On Mon, Nov 09, 2020 at 09:49:31AM -0500, Liang, Kan wrote:
> >> > >
> >> > >> - When the large PEBS was introduced (9c964efa4330), the sched_task()
> >> > >> should
> >> > >> be invoked to flush the PEBS buffer in each context switch. However,
> >> > >> The
> >> > >> perf_sched_events in account_event() is not updated accordingly. The
> >> > >> perf_event_task_sched_* never be invoked for a pure per-CPU context.
> >> > >> Only
> >> > >> per-task event works.
> >> > >> At that time, the perf_pmu_sched_task() is outside of
> >> > >> perf_event_context_sched_in/out. It means that perf has to double
> >> > >> perf_pmu_disable() for per-task event.
> >> > >
> >> > >> - The patch 1 tries to fix broken per-CPU events. The CPU context
> >> > >> cannot be
> >> > >> retrieved from the task->perf_event_ctxp. So it has to be tracked in
> >> > >> the
> >> > >> sched_cb_list. Yes, the code is very similar to the original codes,
> >> > >> but it
> >> > >> is actually the new code for per-CPU events. The optimization for
> >> > >> per-task
> >> > >> events is still kept.
> >> > >> For the case, which has both a CPU context and a task context,
> >> > >> yes, the
> >> > >> __perf_pmu_sched_task() in this patch is not invoked. Because the
> >> > >> sched_task() only need to be invoked once in a context switch. The
> >> > >> sched_task() will be eventually invoked in the task context.
> >> > >
> >> > > The thing is; your first two patches rely on PERF_ATTACH_SCHED_CB and
> >> > > only set that for large pebs. Are you sure the other users (Intel LBR
> >> > > and PowerPC BHRB) don't need it?
> >> >
> >> > I didn't set it for LBR, because the perf_sched_events is always enabled
> >> > for LBR. But, yes, we should explicitly set the PERF_ATTACH_SCHED_CB
> >> > for LBR.
> >> >
> >> > if (has_branch_stack(event))
> >> > inc = true;
> >> >
> >> > >
> >> > > If they indeed do not require the pmu::sched_task() callback for CPU
> >> > > events, then I still think the whole perf_sched_cb_{inc,dec}()
> >> > > interface
> >> >
> >> > No, LBR requires the pmu::sched_task() callback for CPU events.
> >> >
> >> > Now, The LBR registers have to be reset in sched in even for CPU events.
> >> >
> >> > To fix the shorter LBR callstack issue for CPU events, we also need to
> >> > save/restore LBRs in pmu::sched_task().
> >> > https://lore.kernel.org/lkml/1578495789-95006-4-git-send-email-kan.li...@linux.intel.com/
> >> >
> >> > > is confusing at best.
> >> > >
> >> > > Can't we do something like this instead?
> >> > >
> >> > I think the below patch may have two issues.
> >> > - PERF_ATTACH_SCHED_CB is required for LBR (maybe PowerPC BHRB as well)
> >> > now.
> >> > - We may disable the large PEBS later if not all PEBS events support
> >> > large PEBS. The PMU need a way to notify the generic code to decrease
> >> > the nr_sched_task.
> >>
> >> Any updates on this? I've reviewed and tested Kan's patches
> >> and they all look good.
> >>
> >> Maybe we can talk to PPC folks to confirm the BHRB case?
> >
> > Can we move this forward? I saw patch 3/3 also adds PERF_ATTACH_SCHED_CB
> > for PowerPC too. But it'd be nice if ppc folks can confirm the change.
>
> Sorry I've read the whole thread, but I'm still not entirely sure I
> understand the question.
Thanks for your time and sorry about not being clear enough.
We found per-cpu events are not calling pmu::sched_task()
on context switches. So PERF_ATTACH_SCHED_CB was
added to indicate the core logic that it needs to invoke the
callback.
The patch 3/3 added the flag to PPC (for BHRB) with other
changes (I think it should be split like in the patch 2/3) and
want to get ACKs from the PPC folks.
Thanks,
Namhyung
