Re: [PATCH] btrfs: Relax memory barrier in btrfs_tree_unlock
On Wed, Feb 14, 2018 at 02:37:26PM +0200, Nikolay Borisov wrote: > When performing an unlock on an extent buffer we'd like to order the > decrement of extent_buffer::blocking_writers with waking up any > waiters. In such situations it's sufficient to use smp_mb__after_atomic > rather than the heavy smp_mb. On architectures where atomic operations > are fully ordered (such as x86 or s390) unconditionally executing > a heavyweight smp_mb instruction causes a severe hit to performance > while bringin no improvements in terms of correctness. > > The better thing is to use the appropriate smp_mb__after_atomic routine > which will do the correct thing (invoke a full smp_mb or in the case > of ordered atomics insert a compiler barrier). Put another way, > an RMW atomic op + smp_load__after_atomic equals, in terms of > semantics, to a full smp_mb. This ensures that none of the problems > described in the accompanying comment of waitqueue_active occur. > No functional changes. > > Signed-off-by: Nikolay Borisov Reviewed-by: David Sterba -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] btrfs: Relax memory barrier in btrfs_tree_unlock
On 24.02.2018 02:14, David Sterba wrote: > On Wed, Feb 14, 2018 at 02:37:26PM +0200, Nikolay Borisov wrote: >> When performing an unlock on an extent buffer we'd like to order the >> decrement of extent_buffer::blocking_writers with waking up any >> waiters. In such situations it's sufficient to use smp_mb__after_atomic >> rather than the heavy smp_mb. On architectures where atomic operations >> are fully ordered (such as x86 or s390) unconditionally executing >> a heavyweight smp_mb instruction causes a severe hit to performance >> while bringin no improvements in terms of correctness. > > Have you measured this severe performance hit? There is an impact, but I > doubt you'll ever notice it in the profiles given where the > btrfs_tree_unlock appears. Admittedly I haven't :) But I'd say "every little bit helps" > >> The better thing is to use the appropriate smp_mb__after_atomic routine >> which will do the correct thing (invoke a full smp_mb or in the case >> of ordered atomics insert a compiler barrier). Put another way, >> an RMW atomic op + smp_load__after_atomic equals, in terms of >> semantics, to a full smp_mb. This ensures that none of the problems >> described in the accompanying comment of waitqueue_active occur. >> No functional changes. > > I tend to agree. > -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: [PATCH] btrfs: Relax memory barrier in btrfs_tree_unlock
On Wed, Feb 14, 2018 at 02:37:26PM +0200, Nikolay Borisov wrote: > When performing an unlock on an extent buffer we'd like to order the > decrement of extent_buffer::blocking_writers with waking up any > waiters. In such situations it's sufficient to use smp_mb__after_atomic > rather than the heavy smp_mb. On architectures where atomic operations > are fully ordered (such as x86 or s390) unconditionally executing > a heavyweight smp_mb instruction causes a severe hit to performance > while bringin no improvements in terms of correctness. Have you measured this severe performance hit? There is an impact, but I doubt you'll ever notice it in the profiles given where the btrfs_tree_unlock appears. > The better thing is to use the appropriate smp_mb__after_atomic routine > which will do the correct thing (invoke a full smp_mb or in the case > of ordered atomics insert a compiler barrier). Put another way, > an RMW atomic op + smp_load__after_atomic equals, in terms of > semantics, to a full smp_mb. This ensures that none of the problems > described in the accompanying comment of waitqueue_active occur. > No functional changes. I tend to agree. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
