From: Sahitya Tummala <stummala@codeaurora.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Polakov <apolyakov@beget.ru>,
Vladimir Davydov <vdavydov.dev@gmail.com>,
Jan Kara <jack@suse.cz>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH] mm/list_lru.c: use cond_resched_lock() for nlru->lock
Date: Fri, 16 Jun 2017 20:14:00 +0530 [thread overview]
Message-ID: <3c478a65-6cd1-0ee9-2470-7ca368dd88bf@codeaurora.org> (raw)
In-Reply-To: <20170615140523.76f8fc3ca21dae3704f06a56@linux-foundation.org>
On 6/16/2017 2:35 AM, Andrew Morton wrote:
> diff --git a/mm/list_lru.c b/mm/list_lru.c
>> index 5d8dffd..1af0709 100644
>> --- a/mm/list_lru.c
>> +++ b/mm/list_lru.c
>> @@ -249,6 +249,8 @@ restart:
>> default:
>> BUG();
>> }
>> + if (cond_resched_lock(&nlru->lock))
>> + goto restart;
>> }
>>
>> spin_unlock(&nlru->lock);
> This is rather worrying.
>
> a) Why are we spending so long holding that lock that this is occurring?
At the time of crash I see that __list_lru_walk_one() shows number of
entries isolated as 1774475 with nr_items still pending as 130748. On my
system, I see that for dentries of 100000, it takes around 75ms for
__list_lru_walk_one() to complete. So for a total of 1900000 dentries as
in issue scenario, it will take upto 1425ms, which explains why the spin
lockup condition got hit on the other CPU.
It looks like __list_lru_walk_one() is expected to take more time if
there are more number of dentries present. And I think it is a valid
scenario to have those many number dentries.
> b) With this patch, we're restarting the entire scan. Are there
> situations in which this loop will never terminate, or will take a
> very long time? Suppose that this process is getting rescheds
> blasted at it for some reason?
In the above scenario, I observed that the dentry entries from lru list
are removedall the time i.e LRU_REMOVED is returned from the
isolate (dentry_lru_isolate()) callback. I don't know if there is any case
where we skip several entries in the lru list and restartseveral times due
to this cond_resched_lock(). This can happen even with theexisting code
if LRU_RETRY is returned often from the isolate callback.
> IOW this looks like a bit of a band-aid and a deeper analysis and
> understanding might be needed.
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Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
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next prev parent reply other threads:[~2017-06-16 14:44 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-12 0:47 [PATCH] mm/list_lru.c: use cond_resched_lock() for nlru->lock Sahitya Tummala
2017-06-12 13:11 ` Jan Kara
2017-06-15 21:05 ` Andrew Morton
2017-06-16 14:44 ` Sahitya Tummala [this message]
2017-06-17 11:14 ` Vladimir Davydov
2017-06-20 2:52 ` Sahitya Tummala
2017-06-21 6:39 ` [PATCH v2] fs/dcache.c: fix spin lockup issue on nlru->lock Sahitya Tummala
2017-06-21 16:31 ` Vladimir Davydov
2017-06-22 16:31 ` Sahitya Tummala
2017-06-22 17:49 ` Vladimir Davydov
2017-06-28 6:07 ` [PATCH v3 1/2] mm/list_lru.c: fix list_lru_count_node() to be race free Sahitya Tummala
2017-06-28 6:07 ` [PATCH v3 2/2] fs/dcache.c: fix spin lockup issue on nlru->lock Sahitya Tummala
2017-06-28 17:18 ` [PATCH v3 1/2] mm/list_lru.c: fix list_lru_count_node() to be race free Vladimir Davydov
2017-06-29 3:39 ` [PATCH v4 " Sahitya Tummala
2017-07-01 16:28 ` Vladimir Davydov
2017-06-29 3:39 ` [PATCH v4 2/2] fs/dcache.c: fix spin lockup issue on nlru->lock Sahitya Tummala
2017-06-29 22:48 ` Andrew Morton
2017-06-30 3:16 ` Sahitya Tummala
2017-07-01 16:28 ` Vladimir Davydov
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