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

[Peter Zijlstra]

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.