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

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Message-ID: <20181017183310.GB2603@hirez.programming.kicks-ass.net>
Date:   Wed, 17 Oct 2018 20:33:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "acme@...nel.org" <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Stephane Eranian <eranian@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "megha.dey@...el.com" <megha.dey@...el.com>,
        "frederic@...nel.org" <frederic@...nel.org>
Subject: Re: [RFC][PATCH] perf: Rewrite core context handling

On Wed, Oct 17, 2018 at 07:19:55PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 17, 2018 at 04:43:27PM +0000, Song Liu wrote:
> 
> > > That makes task and cpu contexts wildly different, which will complicate
> > > matters I feel.
> > > 
> > 
> > I think we only need different logic when adding events to the task/cpu 
> > contexts. The ctx_sched_in() and ctx_sched_out() will need some extra
> > logic to filter out events that are not being scheduled (don't schedule
> > events on PMU-a when rotating PMU-b). This logic will be the same for 
> > task and cpu context. The difference is, the CPU context will not have
> > such events, because we never added such event to CPU context. 
> > 
> > Does this make sense? I could try draft a RFC to see how difficult it is. 
> 
> I'm not sure it saves much, if we have multiple per-cpu contexts we get
> to re-introduce the active_ctx_list and loose the simplification for the
> online status.
> 
> Plus that fundamental assymetry -- which would bother my OCD forever
> more :-)

Worse, the whole syscall that installs the events will come apart. The
locking for the two cases is different :/