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Date: Mon, 27 Feb 2017 15:29:06 -0500
From: Tejun Heo <tj@kernel.org>
To: Tahsin Erdogan <tahsin@google.com>
Cc: Michal Hocko <mhocko@kernel.org>, Christoph Lameter <cl@linux.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Chris Wilson <chris@chris-wilson.co.uk>,
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        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        zijun_hu <zijun_hu@htc.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        David Rientjes <rientjes@google.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 3/3] percpu: improve allocation success rate for
 non-GFP_KERNEL callers
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Hello,

On Mon, Feb 27, 2017 at 12:27:08PM -0800, Tahsin Erdogan wrote:
> A better example is the call path below:
> 
> pcpu_alloc+0x68f/0x710
> __alloc_percpu_gfp+0xd/0x10
> __percpu_counter_init+0x55/0xc0
> cfq_pd_alloc+0x3b2/0x4e0
> blkg_alloc+0x187/0x230
> blkg_create+0x489/0x670
> blkg_lookup_create+0x9a/0x230
> blkg_conf_prep+0x1fb/0x240
> __cfqg_set_weight_device.isra.105+0x5c/0x180
> cfq_set_weight_on_dfl+0x69/0xc0
> cgroup_file_write+0x39/0x1c0
> kernfs_fop_write+0x13f/0x1d0
> __vfs_write+0x23/0x120
> vfs_write+0xc2/0x1f0
> SyS_write+0x44/0xb0
> entry_SYSCALL_64_fastpath+0x18/0xad
> 
> A failure in this call path gives grief to tools which are trying to
> configure io
> weights. We see occasional failures happen here shortly after reboots even
> when system is not under any memory pressure. Machines with a lot of cpus
> are obviously more vulnerable.

Ah, absolutely, that's a stupid failure but we should be able to fix
that by making the blkg functions take gfp mask and allocate
accordingly, right?  It'll probably take preallocation tricks because
of locking but should be doable.

Thanks.

-- 
tejun