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

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Message-ID: <20190925083557.GA4553@hirez.programming.kicks-ass.net>
Date:   Wed, 25 Sep 2019 10:35:57 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Waiman Long <longman@...hat.com>, 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 Fri, Sep 13, 2019 at 11:50:43AM +1000, Dave Chinner wrote:
> 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.

I'm very much agreeing here. Lock functions with timeouts are a sign of
horrific design.