linux-kernel - Re: [PATCH 0/5] hugetlbfs: Disable PMD sharing for large systems

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Date:   Fri, 13 Sep 2019 11:50:43 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH 0/5] hugetlbfs: Disable PMD sharing for large systems

On Wed, Sep 11, 2019 at 04:05:32PM +0100, 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.
> 
> To fix this problem while perserving existing behavior as much as
> possible, we need to allow timeout in down_write() and disabling PMD
> sharing when it is taking too long to do so. Since a transaction can
> involving touching multiple huge pages, timing out for each of the huge
> page interactions does not completely solve the problem. So a threshold
> is set to completely disable PMD sharing if too many timeouts happen.
> 
> The first 4 patches of this 5-patch series adds a new
> down_write_timedlock() API which accepts a timeout argument and return
> true is locking is successful or false otherwise. It works more or less
> than a down_write_trylock() but the calling thread may sleep.

Just on general principle, this is a non-starter. If a lock is being
held too long, then whatever the lock is protecting needs fixing.
Adding timeouts to locks and sysctls to tune them is not a viable
solution to address latencies caused by algorithm scalability
issues.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com