From: Peter Zijlstra <peterz@infradead.org>
To: Vince Weaver <vince@deater.net>
Cc: mingo@kernel.org, hpa@zytor.com, anton@samba.org,
mathieu.desnoyers@polymtl.ca, linux-kernel@vger.kernel.org,
michael@ellerman.id.au, paulmck@linux.vnet.ibm.com,
benh@kernel.crashing.org, fweisbec@gmail.com, VICTORK@il.ibm.com,
tglx@linutronix.de, oleg@redhat.com, mikey@neuling.org,
linux-tip-commits@vger.kernel.org
Subject: Re: [tip:perf/core] tools/perf: Add required memory barriers
Date: Wed, 6 Nov 2013 19:24:37 +0100 [thread overview]
Message-ID: <20131106182437.GJ16117@laptop.programming.kicks-ass.net> (raw)
In-Reply-To: <alpine.DEB.2.02.1311061223290.28798@pianoman.cluster.toy>
On Wed, Nov 06, 2013 at 12:31:53PM -0500, Vince Weaver wrote:
> On Wed, 6 Nov 2013, Peter Zijlstra wrote:
>
> > On Wed, Nov 06, 2013 at 03:44:56PM +0100, Peter Zijlstra wrote:
> > > long head = ((__atomic long)pc->data_head).load(memory_order_acquire);
> > >
> > > coupled with:
> > >
> > > ((__atomic long)pc->data_tail).store(tail, memory_order_release);
> > >
> > > might be the 'right' and proper C11 incantations to avoid having to
> > > touch kernel macros; but would obviously require a recent compiler.
> > >
> > > Barring that, I think we're stuck with:
> > >
> > > long head = ACCESS_ONCE(pc->data_head);
> > > smp_rmb();
> > >
> > > ...
> > >
> > > smp_mb();
> > > pc->data_tail = tail;
> > >
> > > And using the right asm goo for the barriers. That said, all these asm
> > > barriers should include a compiler barriers (memory clobber) which
> > > _should_ avoid the worst compiler trickery -- although I don't think it
> > > completely obviates the need for ACCESS_ONCE() -- uncertain there.
> >
> > http://software.intel.com/en-us/articles/single-producer-single-consumer-queue/
> >
> > There's one for icc on x86.
> >
>
> I think the problem here is this really isn't a good interface.
Its _so_ common Intel put it on a website ;-) This is a fairly well
documented 'problem'.
> Most users just want the most recent batch of samples. Something like
>
> char buffer[4096];
> int count;
>
> do {
> count=perf_read_sample_buffer(buffer,4096);
> process_samples(buffer);
> } while(count);
>
> where perf_read_sample_buffer() is a syscall that just copies the current
> valid samples to userspace.
>
> Yes, this is inefficient (requires an extra copy of the values) but the
> kernel then could handle all the SMP/multithread/barrier/locking issues.
>
> How much overhead is really introduced by making a copy?
It would make the current perf-record like thing do 2 copies; one into
userspace, and one back into the kernel for write().
Also, we've (unfortunately) already used the read() implementation of
the perf-fd and I'm fairly sure people will not like adding a special
purpose read-like syscall just for this.
That said, I've no idea how expensive it is, not having actually done
it. I do know people were trying to get rid of the one copy we currently
already do.
> Requiring the user of a kernel interface to have a deep knowledge of
> optimizing compilers, barriers, and CPU memory models is just asking for
> trouble.
It shouldn't be all that hard to put this in a (lgpl) library others can
link to -- that way you can build it once (using GCC).
We'd basically need to lift the proposed smp_load_acquire() and
smp_store_release() into userspace for all relevant architectures and
then have something like:
unsigned long perf_read_sample_buffer(void *mmap, long mmap_size, void *dst, long len)
{
struct perf_event_mmap_page *pc = mmap;
void *data = mmap + page_size;
unsigned long data_size = mmap_size - page_size; /* should be 2^n */
unsigned long tail, head, size, copied = 0;
tail = pc->data_tail;
head = smp_load_acquire(&pc->data_head);
size = (head - tail) & (data_size - 1);
while (len && size) {
unsigned long offset = tail & (data_size - 1);
unsigned long bytes = min(len, data_size - offset);
memcpy(data + offset, dst, bytes);
dst += bytes;
tail += bytes;
copied += bytes;
size -= bytes;
len -= bytes;
}
smp_store_release(&pc->data_tail, tail);
return copied;
}
And presto!
> Especially as this all needs to get documented in the manpage and I'm not
> sure that's possible in a sane fashion.
Given that this is a fairly well documented problem that shouldn't be
too hard.
next prev parent reply other threads:[~2013-11-06 18:24 UTC|newest]
Thread overview: 117+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-10-22 23:54 perf events ring buffer memory barrier on powerpc Michael Neuling
2013-10-23 7:39 ` Victor Kaplansky
2013-10-23 14:19 ` Frederic Weisbecker
2013-10-23 14:25 ` Frederic Weisbecker
2013-10-25 17:37 ` Peter Zijlstra
2013-10-25 20:31 ` Michael Neuling
2013-10-27 9:00 ` Victor Kaplansky
2013-10-28 9:22 ` Peter Zijlstra
2013-10-28 10:02 ` Frederic Weisbecker
2013-10-28 12:38 ` Victor Kaplansky
2013-10-28 13:26 ` Peter Zijlstra
2013-10-28 16:34 ` Paul E. McKenney
2013-10-28 20:17 ` Oleg Nesterov
2013-10-28 20:58 ` Victor Kaplansky
2013-10-29 10:21 ` Peter Zijlstra
2013-10-29 10:30 ` Peter Zijlstra
2013-10-29 10:35 ` Peter Zijlstra
2013-10-29 20:15 ` Oleg Nesterov
2013-10-29 19:27 ` Vince Weaver
2013-10-30 10:42 ` Peter Zijlstra
2013-10-30 11:48 ` James Hogan
2013-10-30 12:48 ` Peter Zijlstra
2013-11-06 13:19 ` [tip:perf/core] tools/perf: Add required memory barriers tip-bot for Peter Zijlstra
2013-11-06 13:50 ` Vince Weaver
2013-11-06 14:00 ` Peter Zijlstra
2013-11-06 14:28 ` Peter Zijlstra
2013-11-06 14:55 ` Vince Weaver
2013-11-06 15:10 ` Peter Zijlstra
2013-11-06 15:23 ` Peter Zijlstra
2013-11-06 14:44 ` Peter Zijlstra
2013-11-06 16:07 ` Peter Zijlstra
2013-11-06 17:31 ` Vince Weaver
2013-11-06 18:24 ` Peter Zijlstra [this message]
2013-11-07 8:21 ` Ingo Molnar
2013-11-07 14:27 ` Vince Weaver
2013-11-07 15:55 ` Ingo Molnar
2013-11-11 16:24 ` Peter Zijlstra
2013-11-11 21:10 ` Ingo Molnar
2013-10-29 21:23 ` perf events ring buffer memory barrier on powerpc Michael Neuling
2013-10-30 9:27 ` Paul E. McKenney
2013-10-30 11:25 ` Peter Zijlstra
2013-10-30 14:52 ` Victor Kaplansky
2013-10-30 15:39 ` Peter Zijlstra
2013-10-30 17:14 ` Victor Kaplansky
2013-10-30 17:44 ` Peter Zijlstra
2013-10-31 6:16 ` Paul E. McKenney
2013-11-01 13:12 ` Victor Kaplansky
2013-11-02 16:36 ` Paul E. McKenney
2013-11-02 17:26 ` Paul E. McKenney
2013-10-31 6:40 ` Paul E. McKenney
2013-11-01 14:25 ` Victor Kaplansky
2013-11-02 17:28 ` Paul E. McKenney
2013-11-01 14:56 ` Peter Zijlstra
2013-11-02 17:32 ` Paul E. McKenney
2013-11-03 14:40 ` Paul E. McKenney
2013-11-03 15:17 ` [RFC] arch: Introduce new TSO memory barrier smp_tmb() Peter Zijlstra
2013-11-03 18:08 ` Linus Torvalds
2013-11-03 20:01 ` Peter Zijlstra
2013-11-03 22:42 ` Paul E. McKenney
2013-11-03 23:34 ` Linus Torvalds
2013-11-04 10:51 ` Paul E. McKenney
2013-11-04 11:22 ` Peter Zijlstra
2013-11-04 16:27 ` Paul E. McKenney
2013-11-04 16:48 ` Peter Zijlstra
2013-11-04 19:11 ` Peter Zijlstra
2013-11-04 19:18 ` Peter Zijlstra
2013-11-04 20:54 ` Paul E. McKenney
2013-11-04 20:53 ` Paul E. McKenney
2013-11-05 14:05 ` Will Deacon
2013-11-05 14:49 ` Paul E. McKenney
2013-11-05 18:49 ` Peter Zijlstra
2013-11-06 11:00 ` Will Deacon
2013-11-06 12:39 ` Peter Zijlstra
2013-11-06 12:51 ` Geert Uytterhoeven
2013-11-06 13:57 ` Peter Zijlstra
2013-11-06 18:48 ` Paul E. McKenney
2013-11-06 19:42 ` Peter Zijlstra
2013-11-07 11:17 ` Will Deacon
2013-11-07 13:36 ` Peter Zijlstra
2013-11-07 23:50 ` Mathieu Desnoyers
2013-11-04 11:05 ` Will Deacon
2013-11-04 16:34 ` Paul E. McKenney
2013-11-03 20:59 ` Benjamin Herrenschmidt
2013-11-03 22:43 ` Paul E. McKenney
2013-11-03 17:07 ` perf events ring buffer memory barrier on powerpc Will Deacon
2013-11-03 22:47 ` Paul E. McKenney
2013-11-04 9:57 ` Will Deacon
2013-11-04 10:52 ` Paul E. McKenney
2013-11-01 16:11 ` Peter Zijlstra
2013-11-02 17:46 ` Paul E. McKenney
2013-11-01 16:18 ` Peter Zijlstra
2013-11-02 17:49 ` Paul E. McKenney
2013-10-30 13:28 ` Victor Kaplansky
2013-10-30 15:51 ` Peter Zijlstra
2013-10-30 18:29 ` Peter Zijlstra
2013-10-30 19:11 ` Peter Zijlstra
2013-10-31 4:33 ` Paul E. McKenney
2013-10-31 4:32 ` Paul E. McKenney
2013-10-31 9:04 ` Peter Zijlstra
2013-10-31 15:07 ` Paul E. McKenney
2013-10-31 15:19 ` Peter Zijlstra
2013-11-01 9:28 ` Paul E. McKenney
2013-11-01 10:30 ` Peter Zijlstra
2013-11-02 15:20 ` Paul E. McKenney
2013-11-04 9:07 ` Peter Zijlstra
2013-11-04 10:00 ` Paul E. McKenney
2013-10-31 9:59 ` Victor Kaplansky
2013-10-31 12:28 ` David Laight
2013-10-31 12:55 ` Victor Kaplansky
2013-10-31 15:25 ` Paul E. McKenney
2013-11-01 16:06 ` Victor Kaplansky
2013-11-01 16:25 ` David Laight
2013-11-01 16:30 ` Victor Kaplansky
2013-11-03 20:57 ` Benjamin Herrenschmidt
2013-11-02 15:46 ` Paul E. McKenney
2013-10-28 19:09 ` Oleg Nesterov
2013-10-29 14:06 ` [tip:perf/urgent] perf: Fix perf ring buffer memory ordering tip-bot for Peter Zijlstra
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=20131106182437.GJ16117@laptop.programming.kicks-ass.net \
--to=peterz@infradead.org \
--cc=VICTORK@il.ibm.com \
--cc=anton@samba.org \
--cc=benh@kernel.crashing.org \
--cc=fweisbec@gmail.com \
--cc=hpa@zytor.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-tip-commits@vger.kernel.org \
--cc=mathieu.desnoyers@polymtl.ca \
--cc=michael@ellerman.id.au \
--cc=mikey@neuling.org \
--cc=mingo@kernel.org \
--cc=oleg@redhat.com \
--cc=paulmck@linux.vnet.ibm.com \
--cc=tglx@linutronix.de \
--cc=vince@deater.net \
/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
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).