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Date:   Mon, 10 Oct 2022 12:14:24 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ravi Bangoria <ravi.bangoria@amd.com>
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        sandipan.das@amd.com, ananth.narayan@amd.com, kim.phillips@amd.com,
        santosh.shukla@amd.com
Subject: Re: [PATCH] perf: Rewrite core context handling
Message-ID: <Y0PwgBz6asJOHSan@hirez.programming.kicks-ass.net>
References: <20220829113347.295-1-ravi.bangoria@amd.com>
 <e1d933d1-4772-1357-9cf1-6e5b842508c8@amd.com>
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On Wed, Sep 07, 2022 at 04:58:49PM +0530, Ravi Bangoria wrote:
> > -static void
> > -ctx_flexible_sched_in(struct perf_event_context *ctx,
> > -		      struct perf_cpu_context *cpuctx)
> > +/* XXX .busy thingy from Peter's patch */
> > +static void ctx_flexible_sched_in(struct perf_event_context *ctx, struct pmu *pmu)
> 
> This one turned out to be very easy. Given that, we iterate over each
> pmu, we can just return error if we fail to schedule any flexible event.
> (It wouldn't be straight forward like this if we needed to implement
> pmu=NULL optimization.)
> 
> ---
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index e0232e0bb74e..923656af73fe 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3751,6 +3751,7 @@ static int merge_sched_in(struct perf_event *event, void *data)
>  			cpc = this_cpu_ptr(event->pmu_ctx->pmu->cpu_pmu_context);
>  			perf_mux_hrtimer_restart(cpc);
>  			group_update_userpage(event);
> +			return -EBUSY;
>  		}
>  	}
>  

I'm afraid this breaks things; consider:

  f79256532682 ("perf/core: fix userpage->time_enabled of inactive events")

I totally hate this -- because it means we *HAVE* to iterate the
inactive events, but alas.