linux-kernel - Re: [PATCH] Add slowpath enter/exit trace events

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Date:   Thu, 23 Nov 2017 15:01:27 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     peter.enderborg@...y.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alex Deucher <alexander.deucher@....com>,
        "David S . Miller" <davem@...emloft.net>,
        Harry Wentland <Harry.Wentland@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tony Cheng <Tony.Cheng@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Pavel Tatashin <pasha.tatashin@...cle.com>
Subject: Re: [PATCH] Add slowpath enter/exit trace events

On Thu 23-11-17 13:36:29, Mel Gorman wrote:
> On Thu, Nov 23, 2017 at 01:25:30PM +0100, Michal Hocko wrote:
> > On Thu 23-11-17 11:43:36, peter.enderborg@...y.com wrote:
> > > From: Peter Enderborg <peter.enderborg@...y.com>
> > > 
> > > The warning of slow allocation has been removed, this is
> > > a other way to fetch that information. But you need
> > > to enable the trace. The exit function also returns
> > > information about the number of retries, how long
> > > it was stalled and failure reason if that happened.
> > 
> > I think this is just too excessive. We already have a tracepoint for the
> > allocation exit. All we need is an entry to have a base to compare with.
> > Another usecase would be to measure allocation latency. Information you
> > are adding can be (partially) covered by existing tracepoints.
> > 
> 
> You can gather that by simply adding a probe to __alloc_pages_slowpath
> (like what perf probe does) and matching the trigger with the existing
> mm_page_alloc points.

I am not sure adding a probe on a production system will fly in many
cases. A static tracepoint would be much easier in that case. But I
agree there are other means to accomplish the same thing. My main point
was to have an easy out-of-the-box way to check latencies. But that is
not something I would really insist on.

-- 
Michal Hocko
SUSE Labs