linux-kernel - Re: [PATCH 2/2] blk-iocost: Use alloc_percpu

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Date:   Fri, 11 Dec 2020 15:13:29 +0800
From:   Baolin Wang <baolin.wang@...ux.alibaba.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] blk-iocost: Use alloc_percpu_gfp() to simplify the
 code

Hi Tejun,

> Hello,
> 
> On Thu, Dec 10, 2020 at 06:56:45PM +0800, Baolin Wang wrote:
>> Use alloc_percpu_gfp() with __GFP_ZERO flag, which can remove
>> some explicit initialization code.
> 
> __GFP_ZERO is implicit for percpu allocations and local[64]_t's initial
> states aren't guaranteed to be all zeros on different archs.

Thanks for teaching me this, at least I did not get this from the 
local_ops Documentation before. Just out of curiosity, these local[64]_t 
variables are also allocated from budy allocator ultimately, why they 
can not be initialized to zeros on some ARCHs with __GFP_ZERO? Could you 
elaborate on about this restriction? Thanks.

By the way, seems the kyber-iosched has the same issue, since the 
'struct kyber_cpu_latency' also contains an atomic_t variable.

	kqd->cpu_latency = alloc_percpu_gfp(struct kyber_cpu_latency,
					    GFP_KERNEL | __GFP_ZERO);
	if (!kqd->cpu_latency)
		goto err_kqd;