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Date:   Tue, 22 Oct 2019 11:43:00 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Alexey Budankov <alexey.budankov@linux.intel.com>
Cc:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Kan Liang <kan.liang@linux.intel.com>,
        Stephane Eranian <eranian@google.com>,
        Ian Rogers <irogers@google.com>,
        Song Liu <songliubraving@fb.com>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 4/4] perf/core,x86: synchronize PMU task contexts on
 optimized context switches
Message-ID: <20191022094300.GL1817@hirez.programming.kicks-ass.net>
References: <f4662ac9-e72e-d141-bead-da07e29f81e8@linux.intel.com>
 <4d6320bb-0d15-0028-aefb-a176c986b8db@linux.intel.com>
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On Tue, Oct 22, 2019 at 09:01:11AM +0300, Alexey Budankov wrote:

>  			swap(ctx->task_ctx_data, next_ctx->task_ctx_data);
>  
> +			/*
> +			 * PMU specific parts of task perf context can require
> +			 * additional synchronization which makes sense only if
> +			 * both next_ctx->task_ctx_data and ctx->task_ctx_data
> +			 * pointers are allocated. As an example of such
> +			 * synchronization see implementation details of Intel
> +			 * LBR call stack data profiling;
> +			 */
> +			if (ctx->task_ctx_data && next_ctx->task_ctx_data)
> +				pmu->sync_task_ctx(next_ctx->task_ctx_data,
> +						   ctx->task_ctx_data);

This still does not check if pmu->sync_task_ctx is set. If any other
arch ever uses task_ctx_data without then also supplying this method
things will go *bang*.

Also, I think I prefer the variant I gave you yesterday:

  https://lkml.kernel.org/r/20191021103745.GF1800@hirez.programming.kicks-ass.net

	if (pmu->swap_task_ctx)
		pmu->swap_task_ctx(ctx, next_ctx);
	else
		swap(ctx->task_ctx_data, next_ctx->task_ctx_data);

That also unconfuses the argument order in your above patch (where you
have to undo thw swap).

Alternatively, since there currently is no other arch using
task_ctx_data, we can make the pmu::swap_task_ctx() thing mandatory when
having it and completely replace the swap(), write it like so:


-	swap(ctx->task_ctx_data, next_ctx->task_ctx_data);
+	if (pmu->swap_task_ctx)
+		pmu->swap_task_ctx(ctx, next_ctx);

Hmm?