From: Peter Zijlstra <peterz@infradead.org> To: Florian Weimer <fweimer@redhat.com> Cc: "Pierre-Loup A. Griffais" <pgriffais@valvesoftware.com>, "Thomas Gleixner" <tglx@linutronix.de>, "André Almeida" <andrealmeid@collabora.com>, linux-kernel@vger.kernel.org, kernel@collabora.com, krisman@collabora.com, shuah@kernel.org, linux-kselftest@vger.kernel.org, rostedt@goodmis.org, ryao@gentoo.org, dvhart@infradead.org, mingo@redhat.com, z.figura12@gmail.com, steven@valvesoftware.com, steven@liquorix.net, malteskarupke@web.de, carlos@redhat.com, adhemerval.zanella@linaro.org, libc-alpha@sourceware.org Subject: Re: 'simple' futex interface [Was: [PATCH v3 1/4] futex: Implement mechanism to wait on any of several futexes] Date: Tue, 3 Mar 2020 14:21:50 +0100 Message-ID: <20200303132150.GD2596@hirez.programming.kicks-ass.net> (raw) In-Reply-To: <87pndth9ur.fsf@oldenburg2.str.redhat.com> On Tue, Mar 03, 2020 at 02:00:12PM +0100, Florian Weimer wrote: > * Peter Zijlstra: > > > So how about we introduce new syscalls: > > > > sys_futex_wait(void *uaddr, unsigned long val, unsigned long flags, ktime_t *timo); > > > > struct futex_wait { > > void *uaddr; > > unsigned long val; > > unsigned long flags; > > }; > > sys_futex_waitv(struct futex_wait *waiters, unsigned int nr_waiters, > > unsigned long flags, ktime_t *timo); > > > > sys_futex_wake(void *uaddr, unsigned int nr, unsigned long flags); > > > > sys_futex_cmp_requeue(void *uaddr1, void *uaddr2, unsigned int nr_wake, > > unsigned int nr_requeue, unsigned long cmpval, unsigned long flags); > > > > Where flags: > > > > - has 2 bits for size: 8,16,32,64 > > - has 2 more bits for size (requeue) ?? > > - has ... bits for clocks > > - has private/shared > > - has numa > > What's the actual type of *uaddr? Does it vary by size (which I assume > is in bits?)? Are there alignment constraints? Yeah, u8, u16, u32, u64 depending on the size specified in flags. Naturally aligned. > These system calls seemed to be type-polymorphic still, which is > problematic for defining a really nice C interface. I would really like > to have a strongly typed interface for this, with a nice struct futex > wrapper type (even if it means that we need four of them). You mean like: futex_wait1(u8 *,...) futex_wait2(u16 *,...) futex_wait4(u32 *,...) etc.. ? I suppose making it 16 or so syscalls (more if we want WAKE_OP or requeue across size) is a bit daft, so yeah, sucks. > Will all architectures support all sizes? If not, how do we probe which > size/flags combinations are supported? Up to the native word size (long), IOW ILP32 will not support u64. Overlapping futexes are expressly forbidden, that is: { u32 var; void *addr = &var; } P0() { futex_wait4(addr,...); } P1() { futex_wait1(addr+1,...); } Will have one of them return something bad. > > For NUMA I propose that when NUMA_FLAG is set, uaddr-4 will be 'int > > node_id', with the following semantics: > > > > - on WAIT, node_id is read and when 0 <= node_id <= nr_nodes, is > > directly used to index into per-node hash-tables. When -1, it is > > replaced by the current node_id and an smp_mb() is issued before we > > load and compare the @uaddr. > > > > - on WAKE/REQUEUE, it is an immediate index. > > Does this mean the first waiter determines the NUMA index, and all > future waiters use the same chain even if they are on different nodes? Every new waiter could (re)set node_id, after all, when its not actually waiting, nobody cares what's in that field. > I think documenting this as a node index would be a mistake. It could > be an arbitrary hint for locating the corresponding kernel data > structures. Nah, it allows explicit placement, after all, we have set_mempolicy() and sched_setaffinity() and all the other NUMA crud so that programs that think they know what they're doing, can do explicit placement. > > Any invalid value with result in EINVAL. > > Using uaddr-4 is slightly tricky with a 64-bit futex value, due to the > need to maintain alignment and avoid padding. Yes, but it works, unlike uaddr+4 :-) Also, 1 and 2 byte futexes and NUMA_FLAG are incompatible due to this, but I feel short futexes and NUMA don't really make sense anyway, the only reason to use a short futex is to save space, so you don't want another 4 bytes for numa on top of that anyway.
next prev parent reply index Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-02-13 21:45 [PATCH v3 0/4] Implement FUTEX_WAIT_MULTIPLE operation André Almeida 2020-02-13 21:45 ` [PATCH v3 1/4] futex: Implement mechanism to wait on any of several futexes André Almeida 2020-02-28 19:07 ` Peter Zijlstra 2020-02-28 19:49 ` Peter Zijlstra 2020-02-28 21:25 ` Thomas Gleixner 2020-02-29 0:29 ` Pierre-Loup A. Griffais 2020-02-29 10:27 ` Thomas Gleixner 2020-03-03 2:47 ` Pierre-Loup A. Griffais 2020-03-03 12:00 ` 'simple' futex interface [Was: [PATCH v3 1/4] futex: Implement mechanism to wait on any of several futexes] Peter Zijlstra 2020-03-03 13:00 ` Florian Weimer 2020-03-03 13:21 ` Peter Zijlstra [this message] 2020-03-03 13:47 ` Florian Weimer 2020-03-03 15:01 ` Peter Zijlstra 2020-03-05 16:14 ` André Almeida 2020-03-05 16:25 ` Florian Weimer 2020-03-05 18:51 ` Peter Zijlstra 2020-03-06 16:57 ` David Laight 2020-02-13 21:45 ` [PATCH v3 2/4] selftests: futex: Add FUTEX_WAIT_MULTIPLE timeout test André Almeida 2020-02-13 21:45 ` [PATCH v3 3/4] selftests: futex: Add FUTEX_WAIT_MULTIPLE wouldblock test André Almeida 2020-02-13 21:45 ` [PATCH v3 4/4] selftests: futex: Add FUTEX_WAIT_MULTIPLE wake up test André Almeida 2020-02-19 16:27 ` [PATCH v3 0/4] Implement FUTEX_WAIT_MULTIPLE operation shuah
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=20200303132150.GD2596@hirez.programming.kicks-ass.net \ --to=peterz@infradead.org \ --cc=adhemerval.zanella@linaro.org \ --cc=andrealmeid@collabora.com \ --cc=carlos@redhat.com \ --cc=dvhart@infradead.org \ --cc=fweimer@redhat.com \ --cc=kernel@collabora.com \ --cc=krisman@collabora.com \ --cc=libc-alpha@sourceware.org \ --cc=linux-kernel@vger.kernel.org \ --cc=linux-kselftest@vger.kernel.org \ --cc=malteskarupke@web.de \ --cc=mingo@redhat.com \ --cc=pgriffais@valvesoftware.com \ --cc=rostedt@goodmis.org \ --cc=ryao@gentoo.org \ --cc=shuah@kernel.org \ --cc=steven@liquorix.net \ --cc=steven@valvesoftware.com \ --cc=tglx@linutronix.de \ --cc=z.figura12@gmail.com \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: link
Linux-kselftest Archive on lore.kernel.org Archives are clonable: git clone --mirror https://lore.kernel.org/linux-kselftest/0 linux-kselftest/git/0.git # If you have public-inbox 1.1+ installed, you may # initialize and index your mirror using the following commands: public-inbox-init -V2 linux-kselftest linux-kselftest/ https://lore.kernel.org/linux-kselftest \ linux-kselftest@vger.kernel.org public-inbox-index linux-kselftest Example config snippet for mirrors Newsgroup available over NNTP: nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kselftest AGPL code for this site: git clone https://public-inbox.org/public-inbox.git