From: Peter Zijlstra <peterz@infradead.org> To: "Christoph Müllner" <christophm30@gmail.com> Cc: Palmer Dabbelt <palmer@dabbelt.com>, Anup Patel <anup@brainfault.org>, Guo Ren <guoren@kernel.org>, linux-riscv <linux-riscv@lists.infradead.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Guo Ren <guoren@linux.alibaba.com>, catalin.marinas@arm.com, will.deacon@arm.com, Arnd Bergmann <arnd@arndb.de> Subject: Re: [PATCH] riscv: locks: introduce ticket-based spinlock implementation Date: Tue, 13 Apr 2021 10:03:01 +0200 [thread overview] Message-ID: <YHVQNSfblP6G0Kgl@hirez.programming.kicks-ass.net> (raw) In-Reply-To: <CAHB2gtS6x25Oquf6W4Hhh-diUuZk1GJHTD2DjrffHo93nWbUYw@mail.gmail.com> On Mon, Apr 12, 2021 at 11:54:55PM +0200, Christoph Müllner wrote: > On Mon, Apr 12, 2021 at 7:33 PM Palmer Dabbelt <palmer@dabbelt.com> wrote: > > My plan is to add a generic ticket-based lock, which can be selected at > > compile time. It'll have no architecture dependencies (though it'll > > likely have some hooks for architectures that can make this go faster). > > Users can then just pick which spinlock flavor they want, with the idea > > being that smaller systems will perform better with ticket locks and > > larger systems will perform better with queued locks. The main goal > > here is to give the less widely used architectures an easy way to have > > fair locks, as right now we've got a lot of code duplication because any > > architecture that wants ticket locks has to do it themselves. > > In the case of LL/SC sequences, we have a maximum of 16 instructions > on RISC-V. My concern with a pure-C implementation would be that > we cannot guarantee this (e.g. somebody wants to compile with -O0) > and I don't know of a way to abort the build in case this limit exceeds. > Therefore I have preferred inline assembly for OpenSBI (my initial idea > was to use closure-like LL/SC macros, where you can write the loop > in form of C code). For ticket locks you really only needs atomic_fetch_add() and smp_store_release() and an architectural guarantees that the atomic_fetch_add() has fwd progress under contention and that a sub-word store (through smp_store_release()) will fail the SC. Then you can do something like: void lock(atomic_t *lock) { u32 val = atomic_fetch_add(1<<16, lock); /* SC, gives us RCsc */ u16 ticket = val >> 16; for (;;) { if (ticket == (u16)val) break; cpu_relax(); val = atomic_read_acquire(lock); } } void unlock(atomic_t *lock) { u16 *ptr = (u16 *)lock + (!!__BIG_ENDIAN__); u32 val = atomic_read(lock); smp_store_release(ptr, (u16)val + 1); } That's _almost_ as simple as a test-and-set :-) It isn't quite optimal on x86 for not being allowed to use a memop on unlock, since its being forced into a load-store because of all the volatile, but whatever.
WARNING: multiple messages have this Message-ID (diff)
From: Peter Zijlstra <peterz@infradead.org> To: "Christoph Müllner" <christophm30@gmail.com> Cc: Palmer Dabbelt <palmer@dabbelt.com>, Anup Patel <anup@brainfault.org>, Guo Ren <guoren@kernel.org>, linux-riscv <linux-riscv@lists.infradead.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Guo Ren <guoren@linux.alibaba.com>, catalin.marinas@arm.com, will.deacon@arm.com, Arnd Bergmann <arnd@arndb.de> Subject: Re: [PATCH] riscv: locks: introduce ticket-based spinlock implementation Date: Tue, 13 Apr 2021 10:03:01 +0200 [thread overview] Message-ID: <YHVQNSfblP6G0Kgl@hirez.programming.kicks-ass.net> (raw) In-Reply-To: <CAHB2gtS6x25Oquf6W4Hhh-diUuZk1GJHTD2DjrffHo93nWbUYw@mail.gmail.com> On Mon, Apr 12, 2021 at 11:54:55PM +0200, Christoph Müllner wrote: > On Mon, Apr 12, 2021 at 7:33 PM Palmer Dabbelt <palmer@dabbelt.com> wrote: > > My plan is to add a generic ticket-based lock, which can be selected at > > compile time. It'll have no architecture dependencies (though it'll > > likely have some hooks for architectures that can make this go faster). > > Users can then just pick which spinlock flavor they want, with the idea > > being that smaller systems will perform better with ticket locks and > > larger systems will perform better with queued locks. The main goal > > here is to give the less widely used architectures an easy way to have > > fair locks, as right now we've got a lot of code duplication because any > > architecture that wants ticket locks has to do it themselves. > > In the case of LL/SC sequences, we have a maximum of 16 instructions > on RISC-V. My concern with a pure-C implementation would be that > we cannot guarantee this (e.g. somebody wants to compile with -O0) > and I don't know of a way to abort the build in case this limit exceeds. > Therefore I have preferred inline assembly for OpenSBI (my initial idea > was to use closure-like LL/SC macros, where you can write the loop > in form of C code). For ticket locks you really only needs atomic_fetch_add() and smp_store_release() and an architectural guarantees that the atomic_fetch_add() has fwd progress under contention and that a sub-word store (through smp_store_release()) will fail the SC. Then you can do something like: void lock(atomic_t *lock) { u32 val = atomic_fetch_add(1<<16, lock); /* SC, gives us RCsc */ u16 ticket = val >> 16; for (;;) { if (ticket == (u16)val) break; cpu_relax(); val = atomic_read_acquire(lock); } } void unlock(atomic_t *lock) { u16 *ptr = (u16 *)lock + (!!__BIG_ENDIAN__); u32 val = atomic_read(lock); smp_store_release(ptr, (u16)val + 1); } That's _almost_ as simple as a test-and-set :-) It isn't quite optimal on x86 for not being allowed to use a memop on unlock, since its being forced into a load-store because of all the volatile, but whatever. _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv
next prev parent reply other threads:[~2021-04-13 8:04 UTC|newest] Thread overview: 98+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-03-24 10:14 [PATCH] riscv: locks: introduce ticket-based spinlock implementation guoren 2021-03-24 10:14 ` guoren 2021-03-24 11:09 ` Peter Zijlstra 2021-03-24 11:09 ` Peter Zijlstra 2021-03-24 12:10 ` Guo Ren 2021-03-24 12:10 ` Guo Ren [not found] ` <CAM4kBBK7_s9U2vJbq68yC8WdDEfPQTaCOvn1xds3Si5B-Wpw+A@mail.gmail.com> 2021-03-24 12:23 ` Peter Zijlstra 2021-03-24 12:23 ` Peter Zijlstra 2021-03-24 12:24 ` Guo Ren 2021-03-24 12:24 ` Guo Ren 2021-03-24 12:31 ` Peter Zijlstra 2021-03-24 12:31 ` Peter Zijlstra 2021-03-24 12:28 ` Anup Patel 2021-03-24 12:28 ` Anup Patel 2021-03-24 12:37 ` Peter Zijlstra 2021-03-24 12:37 ` Peter Zijlstra 2021-03-24 12:53 ` Anup Patel 2021-03-24 12:53 ` Anup Patel 2021-04-11 21:11 ` Palmer Dabbelt 2021-04-11 21:11 ` Palmer Dabbelt 2021-04-12 13:32 ` Christoph Müllner 2021-04-12 13:32 ` Christoph Müllner 2021-04-12 14:51 ` Peter Zijlstra 2021-04-12 14:51 ` Peter Zijlstra 2021-04-12 21:21 ` Christoph Müllner 2021-04-12 21:21 ` Christoph Müllner 2021-04-12 17:33 ` Palmer Dabbelt 2021-04-12 17:33 ` Palmer Dabbelt 2021-04-12 21:54 ` Christoph Müllner 2021-04-12 21:54 ` Christoph Müllner 2021-04-13 8:03 ` Peter Zijlstra [this message] 2021-04-13 8:03 ` Peter Zijlstra 2021-04-13 8:17 ` Peter Zijlstra 2021-04-13 8:17 ` Peter Zijlstra 2021-04-14 2:26 ` Guo Ren 2021-04-14 2:26 ` Guo Ren 2021-04-14 7:08 ` Peter Zijlstra 2021-04-14 7:08 ` Peter Zijlstra 2021-04-14 9:05 ` Peter Zijlstra 2021-04-14 9:05 ` Peter Zijlstra 2021-04-14 10:16 ` [RFC][PATCH] locking: Generic ticket-lock Peter Zijlstra 2021-04-14 10:16 ` Peter Zijlstra 2021-04-14 12:39 ` Guo Ren 2021-04-14 12:39 ` Guo Ren 2021-04-14 12:55 ` Peter Zijlstra 2021-04-14 12:55 ` Peter Zijlstra 2021-04-14 13:08 ` Peter Zijlstra 2021-04-14 13:08 ` Peter Zijlstra 2021-04-14 15:59 ` David Laight 2021-04-14 15:59 ` David Laight 2021-04-14 12:45 ` Peter Zijlstra 2021-04-14 12:45 ` Peter Zijlstra 2021-04-14 21:02 ` Stafford Horne 2021-04-14 21:02 ` Stafford Horne 2021-04-14 20:47 ` Stafford Horne 2021-04-14 20:47 ` Stafford Horne 2021-04-15 8:09 ` Peter Zijlstra 2021-04-15 8:09 ` Peter Zijlstra 2021-04-15 9:02 ` Catalin Marinas 2021-04-15 9:02 ` Catalin Marinas 2021-04-15 9:22 ` Will Deacon 2021-04-15 9:22 ` Will Deacon 2021-04-15 9:24 ` Peter Zijlstra 2021-04-15 9:24 ` Peter Zijlstra 2021-04-19 17:35 ` Will Deacon 2021-04-19 17:35 ` Will Deacon 2021-04-23 6:44 ` Palmer Dabbelt 2021-04-23 6:44 ` Palmer Dabbelt 2021-04-13 9:22 ` [PATCH] riscv: locks: introduce ticket-based spinlock implementation Christoph Müllner 2021-04-13 9:22 ` Christoph Müllner 2021-04-13 9:30 ` Catalin Marinas 2021-04-13 9:30 ` Catalin Marinas 2021-04-13 9:55 ` Christoph Müllner 2021-04-13 9:55 ` Christoph Müllner 2021-04-14 0:23 ` Guo Ren 2021-04-14 0:23 ` Guo Ren 2021-04-14 9:17 ` Catalin Marinas 2021-04-14 9:17 ` Catalin Marinas 2021-04-13 9:35 ` Peter Zijlstra 2021-04-13 9:35 ` Peter Zijlstra 2021-04-13 10:25 ` Christoph Müllner 2021-04-13 10:25 ` Christoph Müllner 2021-04-13 10:45 ` Catalin Marinas 2021-04-13 10:45 ` Catalin Marinas 2021-04-13 10:54 ` David Laight 2021-04-13 10:54 ` David Laight 2021-04-14 5:54 ` Guo Ren 2021-04-14 5:54 ` Guo Ren 2021-04-13 11:04 ` Christoph Müllner 2021-04-13 11:04 ` Christoph Müllner 2021-04-13 13:19 ` Guo Ren 2021-04-13 13:19 ` Guo Ren 2021-09-19 16:53 guoren 2021-09-19 16:53 ` guoren 2021-09-25 14:47 ` Guo Ren 2021-09-25 14:47 ` Guo Ren 2021-10-21 13:13 ` Peter Zijlstra 2021-10-21 13:13 ` Peter Zijlstra
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