Messages in this thread | | | Subject | Re: [PATCH] locking/rwsem: Take read lock immediate if empty queue with no writer | From | Waiman Long <> | Date | Fri, 13 Jul 2018 14:32:45 -0400 |
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On 07/13/2018 06:02 AM, Will Deacon wrote: > On Tue, Jul 10, 2018 at 02:31:30PM -0400, Waiman Long wrote: >> It was found that a constant stream of readers might cause the count to >> go negative most of the time after an initial trigger by a writer even >> if no writer was present afterward. As a result, most of the readers >> would have to go through the slowpath reducing their performance. >> >> To avoid that from happening, an additional check is added to detect >> the special case that the reader in the critical section is the only >> one in the wait queue and no writer is present. When that happens, it >> can just have the lock and return immediately without further action. >> Other incoming readers won't see a waiter is present and be forced >> into the slowpath. >> >> After the list_empty() calls, the CPU should have the lock cacheline >> anyway, so an additional semaphore count check shouldn't have any >> performance impact. >> >> Signed-off-by: Waiman Long <longman@redhat.com> >> --- >> kernel/locking/rwsem-xadd.c | 16 +++++++++++++++- >> 1 file changed, 15 insertions(+), 1 deletion(-) > This looks ok to me, but it would be nice to include some performance > figures in the commit log. Do you have any? Phrases such as "shouldn't have > any performance impact" and "probably generate better code" don't fill me > with good feelings ;) > > Will I just got the customer testing results back. I had posted a v2 patch with the customer data. I also remove wordings that may cause confusion.
Cheers, Longman
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