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

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Date:   Mon, 27 Feb 2017 11:32:50 -0800
From:   Tahsin Erdogan <tahsin@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
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

>> >
>> > 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.

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.