linux-kernel - Re: [PATCH] bcache: add cond_resched() in __bch_cache

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Date:   Thu, 7 Mar 2019 23:36:18 +0800
From:   Coly Li <colyli@...e.de>
To:     Shile Zhang <shile.zhang@...ux.alibaba.com>
Cc:     Kent Overstreet <kent.overstreet@...il.com>,
        linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bcache: add cond_resched() in __bch_cache_cmp()

On 2019/3/7 11:06 下午, Shile Zhang wrote:
> 
> On 2019/3/7 18:34, Coly Li wrote:
>> On 2019/3/7 1:15 下午, shile.zhang@...ux.alibaba.com wrote:
>>> From: Shile Zhang <shile.zhang@...ux.alibaba.com>
>>>
>>> Read /sys/fs/bcache/<uuid>/cacheN/priority_stats can take very long
>>> time with huge cache after long run.
>>>
>>> Signed-off-by: Shile Zhang <shile.zhang@...ux.alibaba.com>
>> Hi Shile,
>>
>> Do you test your change ? It will be helpful with more performance data
>> (what problem that you improved).
> 
> In case of 960GB SSD cache device, once read of the 'priority_stats'
> costs about 600ms in our test environment.
> 

After the fix, how much time it takes ?


> The perf tool shown that near 50% CPU time consumed by 'sort()', this
> means once sort will hold the CPU near 300ms.
> 
> In our case, the statistics collector reads the 'priority_stats'
> periodically, it will trigger the schedule latency jitters of the
> 
> task which shared same CPU core.
> 

Hmm, it seems you just make the sort slower, and nothing more changes.
Am I right ?


Coly Li


>>
>> Thanks.
>>
>> Coly Li
>>
>>> ---
>>>   drivers/md/bcache/sysfs.c | 1 +
>>>   1 file changed, 1 insertion(+)
>>>
>>> diff --git a/drivers/md/bcache/sysfs.c b/drivers/md/bcache/sysfs.c
>>> index 557a8a3..028fea1 100644
>>> --- a/drivers/md/bcache/sysfs.c
>>> +++ b/drivers/md/bcache/sysfs.c
>>> @@ -897,6 +897,7 @@ static void bch_cache_set_internal_release(struct
>>> kobject *k)
>>>     static int __bch_cache_cmp(const void *l, const void *r)
>>>   {
>>> +    cond_resched();
>>>       return *((uint16_t *)r) - *((uint16_t *)l);
>>>   }
>>>  
>>


-- 

Coly Li