On Mon, Oct 28, 2013 at 02:26:34PM +0100, Peter Zijlstra wrote:
> On Mon, Oct 28, 2013 at 02:38:29PM +0200, Victor Kaplansky wrote:
> > > 2013/10/25 Peter Zijlstra <pet...@infradead.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 <pet...@infradead.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: an...@samba.org
> Cc: b...@kernel.crashing.org
> Cc: Mathieu Desnoyers <mathieu.desnoy...@polymtl.ca>
> Cc: mich...@ellerman.id.au
> Cc: Paul McKenney <paul...@linux.vnet.ibm.com>
> Cc: Michael Neuling <mi...@neuling.org>
> Cc: Frederic Weisbecker <fweis...@gmail.com>
> Reported-by: Victor Kaplansky <vict...@il.ibm.com>
> Tested-by: Victor Kaplansky <vict...@il.ibm.com>
> Signed-off-by: Peter Zijlstra <pet...@infradead.org>
> Link: http://lkml.kernel.org/r/20131025173749.gg19...@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)
Given that both of the kernel's subsequent operations are stores/writes,
doesn't (A) need to be smp_mb()?
Thanx, Paul
> + * 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();
>
_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@lists.ozlabs.org
https://lists.ozlabs.org/listinfo/linuxppc-dev
