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Mon, 7 Jul 2025 11:46:00 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-ThreadId: T47b27644593db411 Date: Mon, 07 Jul 2025 17:45:35 +0200 From: "Arnd Bergmann" To: "John Paul Adrian Glaubitz" , "Arnd Bergmann" , "Thomas Gleixner" , =?UTF-8?Q?Thomas_Wei=C3=9Fschuh?= , "David S . Miller" , "Andreas Larsson" Cc: "Andy Lutomirski" , "Vincenzo Frascino" , shuah , "Anna-Maria Gleixner" , "Frederic Weisbecker" , "John Stultz" , "Stephen Boyd" , "Catalin Marinas" , "Will Deacon" , "Eric Biggers" , sparclinux@vger.kernel.org, linux-kernel@vger.kernel.org Message-Id: <5c479b4d-65f1-466e-a79e-43f6dfc9345c@app.fastmail.com> In-Reply-To: References: <20250707144726.4008707-1-arnd@kernel.org> Subject: Re: [PATCH 1/2] vdso: sparc: stub out custom vdso implementation Content-Type: text/plain Content-Transfer-Encoding: 7bit On Mon, Jul 7, 2025, at 17:22, John Paul Adrian Glaubitz wrote: > Hello Arnd, > > On Mon, 2025-07-07 at 16:46 +0200, Arnd Bergmann wrote: >> Rip out the whole thing and replace it with a minimal stub as we do >> on parisc and uml. This introduces a small performance regression when >> using a libc that is aware of the vdso (glibc-2.29 or higher). > > Can this performance hit quantified in any way? It's trivial to test calling glibc clock_gettime() in a loop on a specific piece of hardware, the difference should largely depend on how long the timer hardware access takes compared to the syscall overhead. On machines that have neither TICK nor STICK clocksource, the simpler version should even be minimally faster, on those that have one of the two, there is an added cost for entering the syscall on every clock_gettime() as we do on architectures without vdso. > And is there previous serious where this change was made for other archs? See https://lore.kernel.org/lkml/2078551b-c0b0-4201-b8d7-1faafa3647e6@app.fastmail.com/#t for the thread that led to this one. There is also commit 5f55e098b8d0 ("parisc: Add 64-bit gettimeofday() and clock_gettime() vDSO functions") that adds similar stubs on parisc. Arnd