linux-kernel - Re: [PATCH v2 3/3] percpu: improve allocation success rate for non-GFP

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Message-ID: <20170227194711.GS26504@dhcp22.suse.cz>
Date:   Mon, 27 Feb 2017 20:47:11 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Tahsin Erdogan <tahsin@...gle.com>
Cc:     Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Roman Pen <r.peniaev@...il.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        zijun_hu <zijun_hu@....com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] percpu: improve allocation success rate for
 non-GFP_KERNEL callers

On Mon 27-02-17 11:32:50, Tahsin Erdogan wrote:
> >> >
> >> > Yes, this prevents adding more pcpu chunks and so cause "atomic" allocations
> >> > to fail more easily.
> >>
> >> Then I fail to see what is the problem you are trying to fix.
> >
> > To be more specific. Could you describe what more can we do in the
> > vmalloc layer for GFP_NOWAIT allocations? They certainly cannot sleep
> > and cannot perform the reclaim so you have to rely on the background
> > work.
> 
> The main problem that I am trying to fix is in percpu.c code. It
> currently doesn't even attempt to call vmalloc() for GFP_NOWAIT
> case. It solely relies on the background allocator to replenish the
> reserves. I would like percpu.c to call __vmalloc(GFP_NOWAIT) inline
> and see whether that succeeds. If that fails, it is fair to fail the
> call.

OK, that wasn't really clean from the patch to me. I guess it would be
much more easier if a preparatory patch did the gfp mask propagation and
then have patch that changes the pcpu allocator the way you need.
 
> For this to work, __vmalloc() should be ready to serve a caller
> that is holding a spinlock. The might_sleep() in alloc_vmap_area()
> basically prevents us calling vmalloc in this context.

-- 
Michal Hocko
SUSE Labs