linux-kernel - Re: [PATCH v4 4/4] kmod: throttle kmod thread limit

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Date:   Mon, 26 Jun 2017 13:38:31 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     "Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:     akpm@...ux-foundation.org, jeyu@...hat.com, shuah@...nel.org,
        rusty@...tcorp.com.au, ebiederm@...ssion.com,
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Subject: Re: [PATCH v4 4/4] kmod: throttle kmod thread limit

On Fri 2017-06-23 12:20:11, Luis R. Rodriguez wrote:
> If we reach the limit of modprobe_limit threads running the next
> request_module() call will fail. The original reason for adding
> a kill was to do away with possible issues with in old circumstances
> which would create a recursive series of request_module() calls.
> 
> We can do better than just be super aggressive and reject calls
> once we've reached the limit by simply making pending callers wait
> until the threshold has been reduced, and then throttling them in,
> one by one.
> 
> This throttling enables requests over the kmod concurrent limit to
> be processed once a pending request completes. Only the first item
> queued up to wait is woken up. The assumption here is once a task
> is woken it will have no other option to also kick the queue to check
> if there are more pending tasks -- regardless of whether or not it
> was successful.
> 
> By throttling and processing only max kmod concurrent tasks we ensure
> we avoid unexpected fatal request_module() calls, and we keep memory
> consumption on module loading to a minimum.
> 
> With x86_64 qemu, with 4 cores, 4 GiB of RAM it takes the following run
> time to run both tests:
> 
> time ./kmod.sh -t 0008
> real    0m16.523s
> user    0m0.879s
> sys     0m8.977s
> 
> time ./kmod.sh -t 0009
> real    0m56.080s
> user    0m0.717s
> sys     0m10.324s
> 
> Signed-off-by: Luis R. Rodriguez <mcgrof@...nel.org>

All the changes look fine to me. They make perfect sense.

Best Regards,
Petr