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Message-ID: <20180401171356.085a2a33@alans-desktop> Date: Sun, 1 Apr 2018 17:13:56 +0100 From: Alan Cox <gnomes@...rguk.ukuu.org.uk> To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com> Cc: Peter Zijlstra <peterz@...radead.org>, "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>, Boqun Feng <boqun.feng@...il.com>, Andy Lutomirski <luto@...capital.net>, Dave Watson <davejwatson@...com>, linux-kernel@...r.kernel.org, linux-api@...r.kernel.org, Paul Turner <pjt@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, Russell King <linux@....linux.org.uk>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>, Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>, Steven Rostedt <rostedt@...dmis.org>, Josh Triplett <josh@...htriplett.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <will.deacon@....com>, Michael Kerrisk <mtk.manpages@...il.com>, Alexander Viro <viro@...iv.linux.org.uk> Subject: Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12) On Tue, 27 Mar 2018 12:05:23 -0400 Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote: > Expose a new system call allowing each thread to register one userspace > memory area to be used as an ABI between kernel and user-space for two > purposes: user-space restartable sequences and quick access to read the > current CPU number value from user-space. What is the *worst* case timing achievable by using the atomics ? What does it do to real time performance requirements ? For cpu_opv you now give an answer but your answer is assuming there isn't another thread actively thrashing the cache or store buffers, and that the user didn't sneakily pass in a page of uncacheable memory (eg framebuffer, or GPU space). I don't see anything that restricts it to cached pages. With that check in place for x86 at least it would probably be ok and I think the sneaky attacks to make it uncacheable would fail becuase you've got the pages locked so trying to give them to an accelerator will block until you are done. I still like the idea it's just the latencies concern me. > Restartable sequences are atomic with respect to preemption > (making it atomic with respect to other threads running on the > same CPU), as well as signal delivery (user-space execution > contexts nested over the same thread). CPU generally means 'big lump with legs on it'. You are not atomic to the same CPU, because that CPU may have 30+ cores with 8 threads per core. It could do with some better terminology (hardware thread, CPU context ?) > In a typical usage scenario, the thread registering the rseq > structure will be performing loads and stores from/to that > structure. It is however also allowed to read that structure > from other threads. The rseq field updates performed by the > kernel provide relaxed atomicity semantics, which guarantee > that other threads performing relaxed atomic reads of the cpu > number cache will always observe a consistent value. So what happens to your API if the kernel atomics get improved ? You are effectively exporting rseq behaviour from private to public. Alan
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