linux-kernel - Re: [RFC PATCH] s390/uaccess: Limit number of retries for cmpxchg_user

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Message-ID: <Y3Z7aq/yqz3WQVsw@osiris>
Date:   Thu, 17 Nov 2022 19:20:26 +0100
From:   Heiko Carstens <hca@...ux.ibm.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
Cc:     Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Nico Boehr <nrb@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] s390/uaccess: Limit number of retries for
 cmpxchg_user_key

On Thu, Nov 17, 2022 at 11:07:45AM +0100, Janis Schoetterl-Glausch wrote:
> cmpxchg_user_key for byte and short values is implemented via a one word
> cmpxchg loop. Give up trying to perform the cmpxchg if it fails too
> often because of contention on the cache line. This ensures that the
> thread cannot become stuck in the kernel.
> 
> Signed-off-by: Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
> ---
> 
> 
> 128 might seem like a small number, but it actually seems to be plenty.
> I could not get it to return EAGAIN with MAX_LOOP being 8 while 248
> vcpus/threads are hammering the same word.
> This could mean that we don't actually need to limit the number of
> retries, but then, I didn't simulate the absolute worst case, where
> the competing threads are running on dedicated cpus.
> 
> 
>  arch/s390/include/asm/uaccess.h | 35 +++++++++++++++++++++++----------
>  1 file changed, 25 insertions(+), 10 deletions(-)

Looks good, also applied to wip/cmpxchg_user_branch.

Thanks!