Re: [PATCH v2 3/3] percpu: improve allocation success rate for non-GFP_KERNEL callers

[Tejun Heo]

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