linux-kernel - Re: [PATCH 1/2] mm: page_alloc.c: Add tracepoints for slowpath

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Message-ID: <20160727184445.GG21859@dhcp22.suse.cz>
Date:	Wed, 27 Jul 2016 20:44:45 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Rik van Riel <riel@...riel.com>
Cc:	Janani Ravichandran <janani.rvchndrn@...il.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, hannes@...pxchg.org,
	vdavydov@...tuozzo.com, vbabka@...e.cz,
	mgorman@...hsingularity.net, kirill.shutemov@...ux.intel.com,
	bywxiaobai@....com, rostedt@...dmis.org
Subject: Re: [PATCH 1/2] mm: page_alloc.c: Add tracepoints for slowpath

On Wed 27-07-16 14:16:22, Rik van Riel wrote:
> On Wed, 2016-07-27 at 18:33 +0200, Michal Hocko wrote:
> > On Wed 27-07-16 10:47:59, Janani Ravichandran wrote:
> > > 
> > > Add tracepoints to the slowpath code to gather some information.
> > > The tracepoints can also be used to find out how much time was
> > > spent in
> > > the slowpath.
> > I do not think this is a right thing to measure.
> > __alloc_pages_slowpath
> > is more a code organization thing. The fast path might perform an
> > expensive operations like zone reclaim (if node_reclaim_mode > 0) so
> > these trace point would miss it.
> 
> It doesn't look like it does. The fast path either
> returns an allocated page to the caller, or calls
> into the slow path.

I must be missing something here but what prevents
__alloc_pages_nodemask->get_page_from_freelist from doing zone_reclaim?
-- 
Michal Hocko
SUSE Labs