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Date: Mon, 28 Oct 2013 14:26:34 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Victor Kaplansky <VICTORK@...ibm.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>,
Anton Blanchard <anton@...ba.org>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
LKML <linux-kernel@...r.kernel.org>,
Linux PPC dev <linuxppc-dev@...abs.org>,
Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
Michael Ellerman <michael@...erman.id.au>,
Michael Neuling <mikey@...ling.org>,
Paul McKenney <paulmck@...ux.vnet.ibm.com>,
Oleg Nesterov <oleg@...hat.com>
Subject: Re: perf events ring buffer memory barrier on powerpc
On Mon, Oct 28, 2013 at 02:38:29PM +0200, Victor Kaplansky wrote:
> > 2013/10/25 Peter Zijlstra <peterz@...radead.org>:
> > > On Wed, Oct 23, 2013 at 03:19:51PM +0100, Frederic Weisbecker wrote:
> > > I would argue for
> > >
> > > READ ->data_tail READ ->data_head
> > > smp_rmb() (A) smp_rmb() (C)
> > > WRITE $data READ $data
> > > smp_wmb() (B) smp_mb() (D)
> > > STORE ->data_head WRITE ->data_tail
> > >
> > > Where A pairs with D, and B pairs with C.
> > >
> > > I don't think A needs to be a full barrier because we won't in fact
> > > write data until we see the store from userspace. So we simply don't
> > > issue the data WRITE until we observe it.
> > >
> > > OTOH, D needs to be a full barrier since it separates the data READ from
> > > the tail WRITE.
> > >
> > > For B a WMB is sufficient since it separates two WRITEs, and for C an
> > > RMB is sufficient since it separates two READs.
<snip>
> I think you have a point :) IMO, memory barrier (A) is superfluous.
> At producer side we need to ensure that "WRITE $data" is not committed
> to memory before "READ ->data_tail" had seen a new value and if the
> old one indicated that there is no enough space for a new entry. All
> this is already guaranteed by control flow dependancy on single CPU -
> writes will not be committed to the memory if read value of
> "data_tail" doesn't specify enough free space in the ring buffer.
>
> Likewise, on consumer side, we can make use of natural data dependency and
> memory ordering guarantee for single CPU and try to replace "smp_mb" by
> a more light-weight "smp_rmb":
>
> READ ->data_tail READ ->data_head
> // ... smp_rmb() (C)
> WRITE $data READ $data
> smp_wmb() (B) smp_rmb() (D)
> READ $header_size
> STORE ->data_head WRITE ->data_tail = $old_data_tail +
> $header_size
>
> We ensure that all $data is read before "data_tail" is written by
> doing "READ $header_size" after all other data is read and we rely on
> natural data dependancy between "data_tail" write and "header_size"
> read.
I'm not entirely sure I get the $header_size trickery; need to think
more on that. But yes, I did consider the other one. However, I had
trouble having no pairing barrier for (D).
ISTR something like Alpha being able to miss the update (for a long
while) if you don't issue the RMB.
Lets add Paul and Oleg to the thread; this is getting far more 'fun'
that it should be ;-)
For completeness; below the patch as I had queued it.
---
Subject: perf: Fix perf ring buffer memory ordering
From: Peter Zijlstra <peterz@...radead.org>
Date: Mon Oct 28 13:55:29 CET 2013
The PPC64 people noticed a missing memory barrier and crufty old
comments in the perf ring buffer code. So update all the comments and
add the missing barrier.
When the architecture implements local_t using atomic_long_t there
will be double barriers issued; but short of introducing more
conditional barrier primitives this is the best we can do.
Cc: anton@...ba.org
Cc: benh@...nel.crashing.org
Cc: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc: michael@...erman.id.au
Cc: Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Michael Neuling <mikey@...ling.org>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Reported-by: Victor Kaplansky <victork@...ibm.com>
Tested-by: Victor Kaplansky <victork@...ibm.com>
Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Link: http://lkml.kernel.org/r/20131025173749.GG19466@laptop.lan
---
include/uapi/linux/perf_event.h | 12 +++++++-----
kernel/events/ring_buffer.c | 29 ++++++++++++++++++++++++++---
2 files changed, 33 insertions(+), 8 deletions(-)
Index: linux-2.6/include/uapi/linux/perf_event.h
===================================================================
--- linux-2.6.orig/include/uapi/linux/perf_event.h
+++ linux-2.6/include/uapi/linux/perf_event.h
@@ -479,13 +479,15 @@ struct perf_event_mmap_page {
/*
* Control data for the mmap() data buffer.
*
- * User-space reading the @data_head value should issue an rmb(), on
- * SMP capable platforms, after reading this value -- see
- * perf_event_wakeup().
+ * User-space reading the @data_head value should issue an smp_rmb(),
+ * after reading this value.
*
* When the mapping is PROT_WRITE the @data_tail value should be
- * written by userspace to reflect the last read data. In this case
- * the kernel will not over-write unread data.
+ * written by userspace to reflect the last read data, after issueing
+ * an smp_mb() to separate the data read from the ->data_tail store.
+ * In this case the kernel will not over-write unread data.
+ *
+ * See perf_output_put_handle() for the data ordering.
*/
__u64 data_head; /* head in the data section */
__u64 data_tail; /* user-space written tail */
Index: linux-2.6/kernel/events/ring_buffer.c
===================================================================
--- linux-2.6.orig/kernel/events/ring_buffer.c
+++ linux-2.6/kernel/events/ring_buffer.c
@@ -87,10 +87,31 @@ static void perf_output_put_handle(struc
goto out;
/*
- * Publish the known good head. Rely on the full barrier implied
- * by atomic_dec_and_test() order the rb->head read and this
- * write.
+ * Since the mmap() consumer (userspace) can run on a different CPU:
+ *
+ * kernel user
+ *
+ * READ ->data_tail READ ->data_head
+ * smp_rmb() (A) smp_rmb() (C)
+ * WRITE $data READ $data
+ * smp_wmb() (B) smp_mb() (D)
+ * STORE ->data_head WRITE ->data_tail
+ *
+ * Where A pairs with D, and B pairs with C.
+ *
+ * I don't think A needs to be a full barrier because we won't in fact
+ * write data until we see the store from userspace. So we simply don't
+ * issue the data WRITE until we observe it.
+ *
+ * OTOH, D needs to be a full barrier since it separates the data READ
+ * from the tail WRITE.
+ *
+ * For B a WMB is sufficient since it separates two WRITEs, and for C
+ * an RMB is sufficient since it separates two READs.
+ *
+ * See perf_output_begin().
*/
+ smp_wmb();
rb->user_page->data_head = head;
/*
@@ -154,6 +175,8 @@ int perf_output_begin(struct perf_output
* Userspace could choose to issue a mb() before updating the
* tail pointer. So that all reads will be completed before the
* write is issued.
+ *
+ * See perf_output_put_handle().
*/
tail = ACCESS_ONCE(rb->user_page->data_tail);
smp_rmb();
--
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