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Date:   Wed, 6 Dec 2023 12:02:02 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Sven Schnelle <svens@linux.ibm.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        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
Message-ID: <20231206110202.GD30174@noisy.programming.kicks-ass.net>
References: <20231205133015.752543-1-svens@linux.ibm.com>
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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?