linux-kernel - Re: [PATCH v2] perf: Rewrite core context handling

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Date:   Thu, 13 Oct 2022 15:37:23 +0530
From:   Ravi Bangoria <ravi.bangoria@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     acme@...nel.org, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org, songliubraving@...com,
        eranian@...gle.com, ak@...ux.intel.com, mark.rutland@....com,
        frederic@...nel.org, maddy@...ux.ibm.com, irogers@...gle.com,
        will@...nel.org, robh@...nel.org, mingo@...hat.com,
        catalin.marinas@....com, ndesaulniers@...gle.com,
        srw@...dewatkins.net, linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        sandipan.das@....com, ananth.narayan@....com, kim.phillips@....com,
        santosh.shukla@....com, Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [PATCH v2] perf: Rewrite core context handling

On 13-Oct-22 2:17 AM, Peter Zijlstra wrote:
> On Wed, Oct 12, 2022 at 02:16:29PM +0200, Peter Zijlstra wrote:
> 
>> That's the intent yeah. But due to not always holding ctx->mutex over
>> put_pmu_ctx() this might be moot. I'm almost through auditing epc usage
>> and I think ctx->lock is sufficient, fingers crossed.
> 
> So the very last epc usage threw a spanner into the works and made
> things complicated.
> 
> Specifically sys_perf_event_open()'s group_leader case uses
> event->pmu_ctx while only holding ctx->mutex. Therefore we can't fully
> let go of ctx->mutex locking and purely rely on ctx->lock.
> 
> Now the good news is that the annoying put_pmu_ctx() without holding
> ctx->mutex case doesn't actually matter here. Since we hold a reference
> on the group_leader (per the filedesc) the event can't go away,
> therefore it must have a pmu_ctx, and then holding ctx->mutex ensures
> the pmu_ctx is stable -- iow it serializes against
> sys_perf_event_open()'s move_group and perf_pmu_migrate_context()
> changing the epc around.
> 
> So we're going with the normal mutex+lock for modification rule, but
> allow the weird put_pmu_ctx() exception.
> 
> I have the below delta.
> 
> I'm hoping we can call this done -- I'm going to see if I can bribe Mark
> to take a look at the arm64 thing soon and then hopefully queue the
> whole thing once -rc1 happens. That should give us a good long soak
> until the next merge window.

Sounds good. Thanks for all the help!

I've glanced through the changes and they looks fine, below are few minor
points.

> + * Specificially, sys_perf_event_open()'s group_leader case depends on
> + * ctx->mutex pinning the configuration. Since we hold a reference on
> + * group_leader (through the filedesc) it can't fo away, therefore it's

typo: can't go away

> -	refcount_t			refcount;
> +	refcount_t			refcount; /* event <-> ctx */

Ok. We need to remove all those // XXX get/put_ctx() from code
which we added to make refcount a pmu_ctx <-> ctx.

> +#define double_list_for_each_entry(pos1, pos2, head1, head2, member)	\
> +	for (pos1 = list_first_entry(head1, typeof(*pos1), member),	\
> +	     pos2 = list_first_entry(head2, typeof(*pos2), member);	\
> +	     !list_entry_is_head(pos1, head1, member) &&		\
> +	     !list_entry_is_head(pos2, head2, member);			\
> +	     pos1 = list_next_entry(pos1, member),			\
> +	     pos2 = list_next_entry(pos2, member))
> +
>  static void perf_event_swap_task_ctx_data(struct perf_event_context *prev_ctx,
>  					  struct perf_event_context *next_ctx)

While this is unrelated to this patch, shouldn't we also need to swap
event->hw.target? A purely hypothetical scenario: Consider two processes
having clone contexts (for example, two children of the same parent).
While process switch between these two, the perf event context would get
swapped but event->hw.target will point to other sibling's task_struct.
If any one process exit just after single context swap, _free_event()
will call put_task_context() on sibling process' task_struct.

> @@ -12436,6 +12463,9 @@ SYSCALL_DEFINE5(perf_event_open,
>  			 * Allow the addition of software events to hw
>  			 * groups, this is safe because software events
>  			 * never fail to schedule.
> +			 *
> +			 * Note the comment that goes with struct
> +			 * pmu_event_pmu_context.

typo: perf_event_pmu_context

The good (or bad? ;)) news is, perf test and Vince's perf_event_tests
are running fine without any regression on my machine.

Thanks,
Ravi