----- Original Message -----
> From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> To: "Mathieu Desnoyers" <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: peterz@xxxxxxxxxxxxx, linux-arch@xxxxxxxxxxxxxxx, geert@xxxxxxxxxxxxxx, torvalds@xxxxxxxxxxxxxxxxxxxx,
> VICTORK@xxxxxxxxxx, oleg@xxxxxxxxxx, anton@xxxxxxxxx, benh@xxxxxxxxxxxxxxxxxxx, fweisbec@xxxxxxxxx,
> michael@xxxxxxxxxxxxxx, mikey@xxxxxxxxxxx, linux@xxxxxxxxxxxxxxxx, schwidefsky@xxxxxxxxxx, "heiko carstens"
> <heiko.carstens@xxxxxxxxxx>, "tony luck" <tony.luck@xxxxxxxxx>
> Sent: Thursday, November 7, 2013 9:19:28 PM
> Subject: Re: [PATCH 4/4] perf: Optimize perf_output_begin() -- weaker memory barrier
>
> On Thu, Nov 07, 2013 at 04:16:17PM -0500, Mathieu Desnoyers wrote:
> > * peterz@xxxxxxxxxxxxx (peterz@xxxxxxxxxxxxx) wrote:
> > > Apply the fancy new smp_load_acquire() and smp_store_release() to
> > > potentially avoid the full memory barrier in perf_output_begin().
> > >
> > > On x86 (and other TSO like architectures) this removes all explicit
> > > memory fences, on weakly ordered systems this often allows the use of
> > > weaker barriers; in particular on powerpc we demote from a full sync
> > > to a cheaper lwsync.
> > >
> > > Cc: Tony Luck <tony.luck@xxxxxxxxx>
> > > Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> > > Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> > > Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> > > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
> > > Cc: Michael Ellerman <michael@xxxxxxxxxxxxxx>
> > > Cc: Michael Neuling <mikey@xxxxxxxxxxx>
> > > Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
> > > Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> > > Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
> > > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> > > Cc: Victor Kaplansky <VICTORK@xxxxxxxxxx>
> > > Suggested-by: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > ---
> > > kernel/events/ring_buffer.c | 62
> > > +++++++++++++++++++++++++-------------------
> > > 1 file changed, 36 insertions(+), 26 deletions(-)
> > >
> > > --- a/kernel/events/ring_buffer.c
> > > +++ b/kernel/events/ring_buffer.c
> > > @@ -41,6 +41,32 @@ static void perf_output_get_handle(struc
> > > handle->wakeup = local_read(&rb->wakeup);
> > > }
> > >
> > > +/*
> > > + * Our user visible data structure (struct perf_event_mmap_page) uses
> > > + * u64 values for ->data_head and ->data_tail to avoid size variance
> > > + * across 32/64 bit.
> > > + *
> > > + * Since you cannot mmap() a buffer larger than your memory address
> > > space
> > > + * we're naturally limited to unsigned long and can avoid writing the
> > > + * high word on 32bit systems (its always 0)
> > > + *
> > > + * This allows us to always use a single load/store.
> > > + */
> > > +#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> > > +static inline unsigned long *low_word(u64 *ptr)
> > > +{
> > > + return (unsigned long *)ptr;
> > > +}
> > > +#else /* __ORDER_BIG_ENDIAN__ */
> > > +static inline unsigned long *low_word(u64 *ptr)
> > > +{
> > > + void *_ptr = ptr;
> > > + _ptr += sizeof(u64);
> > > + _ptr -= sizeof(unsigned long);
> > > + return (unsigned long *)_ptr;
> > > +}
> > > +#endif
> > > +
> > > static void perf_output_put_handle(struct perf_output_handle *handle)
> > > {
> > > struct ring_buffer *rb = handle->rb;
> > > @@ -61,28 +87,15 @@ static void perf_output_put_handle(struc
> > > *
> > > * kernel user
> > > *
> > > - * READ ->data_tail READ ->data_head
> > > - * smp_mb() (A) smp_rmb() (C)
> > > + * READ.acq ->data_tail (A) READ.acq ->data_head (C)
> >
> > I don't get how the barrier() in the TSO implementation of
> > smp_load_acquire (A) orders the following writes to $data after the
> > READ.acq of data_tail. I'm probably missing something.
> >
> > Also, I don't get how the smp_load_acquire (C) with the barrier() (x86
> > TSO) orders READ $data after the READ.acq of data_head.
> >
> > I don't have the TSO model fresh in memory however.
>
> TSO guarantees that earlier reads will not be reordered with later
> writes, so only a barrier() is required.
OK, so this takes care of (A), but how about (C) ? It's about the order of two READ operations.
Thanks,
Mathieu
>
> Thanx, Paul
>
> > > * WRITE $data READ $data
> > > - * smp_wmb() (B) smp_mb() (D)
> > > - * STORE ->data_head WRITE ->data_tail
> > > + * STORE.rel ->data_head (B) WRITE.rel ->data_tail (D)
> >
> > You might want to choose either STORE or WRITE for consistency.
> >
> > Thanks,
> >
> > Mathieu
> >
> > > *
> > > * 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. Be conservative for now.
> > > - *
> > > - * 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;
> > > + smp_store_release(low_word(&rb->user_page->data_head), head);
> > >
> > > /*
> > > * Now check if we missed an update -- rely on previous implied
> > > @@ -139,7 +152,13 @@ int perf_output_begin(struct perf_output
> > > perf_output_get_handle(handle);
> > >
> > > do {
> > > - tail = ACCESS_ONCE(rb->user_page->data_tail);
> > > + tail = smp_load_acquire(low_word(&rb->user_page->data_tail));
> > > + /*
> > > + * STORES of the data below cannot pass the ACQUIRE barrier.
> > > + *
> > > + * Matches with an smp_mb() or smp_store_release() in userspace
> > > + * as described in perf_output_put_handle().
> > > + */
> > > offset = head = local_read(&rb->head);
> > > if (!rb->overwrite &&
> > > unlikely(CIRC_SPACE(head, tail, perf_data_size(rb)) < size))
> > > @@ -147,15 +166,6 @@ int perf_output_begin(struct perf_output
> > > head += size;
> > > } while (local_cmpxchg(&rb->head, offset, head) != offset);
> > >
> > > - /*
> > > - * Separate the userpage->tail read from the data stores below.
> > > - * Matches the MB userspace SHOULD issue after reading the data
> > > - * and before storing the new tail position.
> > > - *
> > > - * See perf_output_put_handle().
> > > - */
> > > - smp_mb();
> > > -
> > > if (unlikely(head - local_read(&rb->wakeup) > rb->watermark))
> > > local_add(rb->watermark, &rb->wakeup);
> > >
> > >
> > >
> >
> > --
> > Mathieu Desnoyers
> > EfficiOS Inc.
> > http://www.efficios.com
> >
>
>
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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