From: Mike Kravetz <mike.kravetz@oracle.com>
To: Waiman Long <longman@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Davidlohr Bueso <dave@stgolabs.net>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH] hugetlbfs: Take read_lock on i_mmap for PMD sharing
Date: Thu, 7 Nov 2019 11:31:56 -0800 [thread overview]
Message-ID: <ae9384e6-ac8d-8dd3-54c4-34d368c9515c@oracle.com> (raw)
In-Reply-To: <20191107190628.22667-1-longman@redhat.com>
On 11/7/19 11:06 AM, Waiman Long wrote:
> A customer with large SMP systems (up to 16 sockets) with application
> that uses large amount of static hugepages (~500-1500GB) are experiencing
> random multisecond delays. These delays was caused by the long time it
> took to scan the VMA interval tree with mmap_sem held.
>
> The sharing of huge PMD does not require changes to the i_mmap at all.
> As a result, we can just take the read lock and let other threads
> searching for the right VMA to share in parallel. Once the right
> VMA is found, either the PMD lock (2M huge page for x86-64) or the
> mm->page_table_lock will be acquired to perform the actual PMD sharing.
>
> Lock contention, if present, will happen in the spinlock. That is much
> better than contention in the rwsem where the time needed to scan the
> the interval tree is indeterminate.
>
> With this patch applied, the customer is seeing significant improvements
> over the unpatched kernel.
Thanks for getting this tested in the customers environment!
> Signed-off-by: Waiman Long <longman@redhat.com>
Just a small typo in the comment, otherwise.
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
> mm/hugetlb.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index b45a95363a84..087e7ff00137 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4842,7 +4842,11 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
> if (!vma_shareable(vma, addr))
> return (pte_t *)pmd_alloc(mm, pud, addr);
>
> - i_mmap_lock_write(mapping);
> + /*
> + * PMD sharing does not require changes to i_mmap. So a read lock
> + * is enuogh.
s/enuogh/enough/
> + */
> + i_mmap_lock_read(mapping);
> vma_interval_tree_foreach(svma, &mapping->i_mmap, idx, idx) {
> if (svma == vma)
> continue;
> @@ -4872,7 +4876,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
> spin_unlock(ptl);
> out:
> pte = (pte_t *)pmd_alloc(mm, pud, addr);
> - i_mmap_unlock_write(mapping);
> + i_mmap_unlock_read(mapping);
> return pte;
> }
>
>
--
Mike Kravetz
next prev parent reply other threads:[~2019-11-07 19:32 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-07 19:06 [PATCH] hugetlbfs: Take read_lock on i_mmap for PMD sharing Waiman Long
2019-11-07 19:13 ` Waiman Long
2019-11-07 19:15 ` Waiman Long
2019-11-07 19:31 ` Mike Kravetz [this message]
2019-11-07 19:42 ` Matthew Wilcox
2019-11-07 21:06 ` Waiman Long
2019-11-07 19:54 ` Matthew Wilcox
2019-11-07 21:27 ` Waiman Long
2019-11-07 21:49 ` Mike Kravetz
2019-11-07 21:56 ` Mike Kravetz
2019-11-08 2:04 ` Davidlohr Bueso
2019-11-08 3:22 ` Andrew Morton
2019-11-08 19:10 ` Mike Kravetz
2019-11-09 1:47 ` Mike Kravetz
2019-11-12 17:27 ` Waiman Long
2019-11-12 23:11 ` Mike Kravetz
2019-11-13 2:55 ` Waiman Long
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=ae9384e6-ac8d-8dd3-54c4-34d368c9515c@oracle.com \
--to=mike.kravetz@oracle.com \
--cc=akpm@linux-foundation.org \
--cc=dave@stgolabs.net \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=longman@redhat.com \
--cc=mingo@redhat.com \
--cc=peterz@infradead.org \
--cc=will.deacon@arm.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).