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Message-ID: <20190213191705.GV9565@techsingularity.net>
Date:   Wed, 13 Feb 2019 19:17:06 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        jolsa@...hat.com
Subject: Re: [PATCH v0 1/2] perf: Add an option to ask for high order
 allocations for AUX buffers

On Wed, Feb 13, 2019 at 06:54:34PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 13, 2019 at 05:47:56PM +0000, Mel Gorman wrote:
> > If there is a tangiable performance benefit from using contiguous regions
> > then I would suggest optimistically allocating them with appropriate
> > GFP flags to avoid large latencies at startup time and fall back if
> > necessary. 
> 
> Right; the code does the fallback thing. It successively tries smaller
> order allocations until 0-order fails.
> 
> It currently uses:
> 
> #define PERF_AUX_GFP    (GFP_KERNEL | __GFP_ZERO | __GFP_NOWARN | __GFP_NORETRY)
> 
> Is that what you could consider appropriate?

Yeah, spot on. Conceivably the semantics of __GFP_NORETY will change a
little at some point in the next year but not in any way I'd consider
harmful (depends on how THP and locality discussions go). Even *if*
we did something harmful, there will be complaints before it's problematic.

-- 
Mel Gorman
SUSE Labs

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