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. -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
WARNING: multiple messages have this Message-ID (diff)
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. -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2017-06-16 14:44 UTC|newest] Thread overview: 38+ 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 0:47 ` Sahitya Tummala 2017-06-12 13:11 ` Jan Kara 2017-06-12 13:11 ` Jan Kara 2017-06-15 21:05 ` Andrew Morton 2017-06-15 21:05 ` Andrew Morton 2017-06-16 14:44 ` Sahitya Tummala [this message] 2017-06-16 14:44 ` Sahitya Tummala 2017-06-17 11:14 ` Vladimir Davydov 2017-06-17 11:14 ` Vladimir Davydov 2017-06-20 2:52 ` Sahitya Tummala 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 6:39 ` Sahitya Tummala 2017-06-21 16:31 ` Vladimir Davydov 2017-06-21 16:31 ` Vladimir Davydov 2017-06-22 16:31 ` Sahitya Tummala 2017-06-22 16:31 ` Sahitya Tummala 2017-06-22 17:49 ` Vladimir Davydov 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 ` 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 6:07 ` 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 2017-06-28 17:18 ` Vladimir Davydov 2017-06-29 3:39 ` [PATCH v4 " Sahitya Tummala 2017-06-29 3:39 ` Sahitya Tummala 2017-07-01 16:28 ` Vladimir Davydov 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 3:39 ` Sahitya Tummala 2017-06-29 22:48 ` Andrew Morton 2017-06-29 22:48 ` Andrew Morton 2017-06-30 3:16 ` Sahitya Tummala 2017-06-30 3:16 ` Sahitya Tummala 2017-07-01 16:28 ` Vladimir Davydov 2017-07-01 16:28 ` Vladimir Davydov
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