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From:   Sven Schnelle <svens@linux.ibm.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        linux-kernel@vger.kernel.org, Heiko Carstens <hca@linux.ibm.com>
Subject: Re: [PATCH 0/3] entry: inline syscall enter/exit functions
In-Reply-To: <20231206110202.GD30174@noisy.programming.kicks-ass.net> (Peter
        Zijlstra's message of "Wed, 6 Dec 2023 12:02:02 +0100")
References: <20231205133015.752543-1-svens@linux.ibm.com>
        <20231206110202.GD30174@noisy.programming.kicks-ass.net>
Date:   Thu, 14 Dec 2023 09:24:17 +0100
Message-ID: <yt9do7etw5se.fsf@linux.ibm.com>
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Peter Zijlstra <peterz@infradead.org> writes:

> On Tue, Dec 05, 2023 at 02:30:12PM +0100, Sven Schnelle wrote:
>> Hi List,
>> 
>> looking into the performance of syscall entry/exit after s390 switched
>> to generic entry showed that there's quite some overhead calling some
>> of the entry/exit work functions even when there's nothing to do.
>> This patchset moves the entry and exit function to entry-common.h, so
>> non inlined code gets only called when there is some work pending.
>
> So per that logic you wouldn't need to inline exit_to_user_mode_loop()
> for example, that's only called when there is a EXIT_TO_USER_MODE_WORK
> bit set.
>
> That is, I'm just being pedantic here and pointing out that your
> justification doesn't cover the extent of the changes.
>
>> I wrote a small program that just issues invalid syscalls in a loop.
>> On an s390 machine, this results in the following numbers:
>> 
>> without this series:
>> 
>> # ./syscall 1000000000
>> runtime: 94.886581s / per-syscall 9.488658e-08s
>> 
>> with this series:
>> 
>> ./syscall 1000000000
>> runtime: 84.732391s / per-syscall 8.473239e-08s
>> 
>> so the time required for one syscall dropped from 94.8ns to
>> 84.7ns, which is a drop of about 11%.
>
> That is obviously very nice, and I don't immediately see anything wrong
> with moving the lot to header based inlines.
>
> Thomas?

Thomas, any opinion on this change?