linux-kernel - Re: [PATCH v6 4/7] arm64: perf: Defer irq_work to IPI_IRQ

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Date:   Wed, 19 Aug 2020 16:23:26 +0200
From:   peterz@...radead.org
To:     Alexandru Elisei <alexandru.elisei@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, maz@...nel.org, will@...nel.org,
        catalin.marinas@....com, swboyd@...omium.org,
        sumit.garg@...aro.org, Julien Thierry <julien.thierry@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Will Deacon <will.deacon@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH v6 4/7] arm64: perf: Defer irq_work to IPI_IRQ_WORK

On Wed, Aug 19, 2020 at 02:34:16PM +0100, Alexandru Elisei wrote:
> From: Julien Thierry <julien.thierry@....com>
> 
> When handling events, armv8pmu_handle_irq() calls perf_event_overflow(),
> and subsequently calls irq_work_run() to handle any work queued by
> perf_event_overflow(). As perf_event_overflow() raises IPI_IRQ_WORK when
> queuing the work, this isn't strictly necessary and the work could be
> handled as part of the IPI_IRQ_WORK handler.
> 
> In the common case the IPI handler will run immediately after the PMU IRQ
> handler, and where the PE is heavily loaded with interrupts other handlers
> may run first, widening the window where some counters are disabled.
> 
> In practice this window is unlikely to be a significant issue, and removing
> the call to irq_work_run() would make the PMU IRQ handler NMI safe in
> addition to making it simpler, so let's do that.

Makes sense, IIRC this code was written before ARM grew IPI_IRQ_WORK
support and then it makes sense, but now that you have it and are moving
to NMI-like context this is absolutely the right thing to do.