Re: [PATCH v5] blkcg: allocate struct blkcg_gq outside request queue spinlock

[Tejun Heo]

On Thu, Mar 09, 2017 at 12:05:31AM -0800, Tahsin Erdogan wrote:
> blkg_conf_prep() currently calls blkg_lookup_create() while holding
> request queue spinlock. This means allocating memory for struct
> blkcg_gq has to be made non-blocking. This causes occasional -ENOMEM
> failures in call paths like 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
> 
> In the code path above, percpu allocator cannot call vmalloc() due to
> queue spinlock.
> 
> A failure in this call path gives grief to tools which are trying to
> configure io weights. We see occasional failures happen shortly after
> reboots even when system is not under any memory pressure. Machines
> with a lot of cpus are more vulnerable to this condition.
> 
> Update blkg_create() function to temporarily drop the rcu and queue
> locks when it is allowed by gfp mask.
> 
> Suggested-by: Tejun Heo <tj@xxxxxxxxxx>
> Signed-off-by: Tahsin Erdogan <tahsin@xxxxxxxxxx>

Acked-by: Tejun Heo <tj@xxxxxxxxxx>

Thanks.

-- 
tejun