linux-kernel - Re: [PATCH v2] locking/rwbase: Prevent indefinite writer starvation

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Date:   Fri, 20 Jan 2023 07:36:41 -0800
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Linux-RT <linux-rt-users@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] locking/rwbase: Prevent indefinite writer starvation

On Fri, 20 Jan 2023, Mel Gorman wrote:

>locking/rwbase: Prevent indefinite writer starvation
>
>rw_semaphore and rwlock are explicitly unfair to writers in the presense
>of readers by design with a PREEMPT_RT configuration. Commit 943f0edb754f
>("locking/rt: Add base code for RT rw_semaphore and rwlock") notes;
>
>        The implementation is writer unfair, as it is not feasible to do
>        priority inheritance on multiple readers, but experience has shown
>        that real-time workloads are not the typical workloads which are
>        sensitive to writer starvation.
>
>While atypical, it's also trivial to block writers with PREEMPT_RT
>indefinitely without ever making forward progress. Since LTP-20220121,
>the dio_truncate test case went from having 1 reader to having 16 readers
>and the number of readers is sufficient to prevent the down_write ever
>succeeding while readers exist. Eventually the test is killed after 30
>minutes as a failure.
>
>dio_truncate is not a realtime application but indefinite writer starvation
>is undesirable. The test case has one writer appending and truncating files
>A and B while multiple readers read file A. The readers and writer are
>contending for one file's inode lock which never succeeds as the readers
>keep reading until the writer is done which never happens.
>
>This patch records a timestamp when the first writer is blocked. DL /
>RT tasks can continue to take the lock for read as long as readers exist
>indefinitely. Other readers can acquire the read lock unless a writer
>has been blocked for a minimum of 4ms. This is sufficient to allow the
>dio_truncate test case to complete within the 30 minutes timeout.
>
>Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>

LGTM (albeit Sebastian's last comment).

Reviewed-by: Davidlohr Bueso <dave@...olabs.net>