linux-kernel - Re: [PATCH RFC 04/10] perf: Introduce deferred user callchains

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Date:   Sat, 11 Nov 2023 10:54:30 -0800
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Indu Bhagat <indu.bhagat@...cle.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
        linux-toolchains@...r.kernel.org
Subject: Re: [PATCH RFC 04/10] perf: Introduce deferred user callchains

On Sat, Nov 11, 2023 at 10:49:10AM -0800, Josh Poimboeuf wrote:
> On Fri, Nov 10, 2023 at 10:57:58PM -0800, Namhyung Kim wrote:
> > > +static void perf_pending_task_unwind(struct perf_event *event)
> > > +{
> > > +       struct pt_regs *regs = task_pt_regs(current);
> > > +       struct perf_output_handle handle;
> > > +       struct perf_event_header header;
> > > +       struct perf_sample_data data;
> > > +       struct perf_callchain_entry *callchain;
> > > +
> > > +       callchain = kmalloc(sizeof(struct perf_callchain_entry) +
> > > +                           (sizeof(__u64) * event->attr.sample_max_stack) +
> > > +                           (sizeof(__u64) * 1) /* one context */,
> > > +                           GFP_KERNEL);
> > 
> > Any chance it can reuse get_perf_callchain() instead of
> > allocating the callchains every time?
> 
> I don't think so, because if it gets preempted, the new task might also
> need to do an unwind.  But there's only one task-context callchain per
> CPU.

BTW it's not just preemption, this code can also block when the unwinder
tries to copy from user space.  So disabling preemption isn't an option.

-- 
Josh