linux-kernel - Re: [PATCH v6] perf: Sharing PMU counters across compatible events

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Message-ID: <20191106085615.GW4114@hirez.programming.kicks-ass.net>
Date:   Wed, 6 Nov 2019 09:56:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     open list <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "acme@...nel.org" <acme@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v6] perf: Sharing PMU counters across compatible events

On Tue, Nov 05, 2019 at 11:51:42PM +0000, Song Liu wrote:

> > We allocate a separate perf_event for perf_event_dup->master. This needs
> > extra attention, because perf_event_alloc() may sleep. To allocate the
> > master event properly, a new pointer, tmp_master, is added to perf_event.
> > tmp_master carries a separate perf_event into list_[add|del]_event().
> > The master event has valid ->ctx and holds ctx->refcount.
> 
> If we do GFP_ATOMIC in perf_event_alloc(), maybe with an extra option, we
> don't need the tmp_master hack. So we only allocate master when we will 
> use it. 

You can't, that's broken on -RT. ctx->lock is a raw_spinlock_t and
allocator locks are spinlock_t.