On 02/15/2015 01:01 AM, Raghavendra K T wrote: > On 02/15/2015 11:25 AM, Raghavendra K T wrote: >> Paravirt spinlock clears slowpath flag after doing unlock. >> As explained by Linus currently it does: >> prev = *lock; >> add_smp(&lock->tickets.head, TICKET_LOCK_INC); >> >> /* add_smp() is a full mb() */ >> >> if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG)) >> __ticket_unlock_slowpath(lock, prev); >> >> which is *exactly* the kind of things you cannot do with spinlocks, >> because after you've done the "add_smp()" and released the spinlock >> for the fast-path, you can't access the spinlock any more. Exactly >> because a fast-path lock might come in, and release the whole data >> structure. >> >> Linus suggested that we should not do any writes to lock after unlock(), >> and we can move slowpath clearing to fastpath lock. >> >> So this patch implements the fix with: >> 1. Moving slowpath flag to head (Oleg): >> Unlocked locks don't care about the slowpath flag; therefore we can keep >> it set after the last unlock, and clear it again on the first (try)lock. >> -- this removes the write after unlock. note that keeping slowpath flag would >> result in unnecessary kicks. >> By moving the slowpath flag from the tail to the head ticket we also avoid >> the need to access both the head and tail tickets on unlock. >> >> 2. use xadd to avoid read/write after unlock that checks the need for >> unlock_kick (Linus): >> We further avoid the need for a read-after-release by using xadd; >> the prev head value will include the slowpath flag and indicate if we >> need to do PV kicking of suspended spinners -- on modern chips xadd >> isn't (much) more expensive than an add + load. >> >> Result: >> setup: 16core (32 cpu +ht sandy bridge 8GB 16vcpu guest) >> benchmark overcommit %improve >> kernbench 1x -0.13 >> kernbench 2x 0.02 >> dbench 1x -1.77 >> dbench 2x -0.63 >> >> [Jeremy: hinted missing TICKET_LOCK_INC for kick] >> [Oleg: Moving slowpath flag to head, ticket_equals idea] >> [PeterZ: Detailed changelog] >> >> Reported-by: Sasha Levin <sasha.levin@xxxxxxxxxx> >> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> >> Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx> >> --- > > Sasha, Hope this addresses invalid read concern you had with latest > xadd based implementation. > > (Think we need to test without Oleg's PATCH] sched/completion: completion_done() should serialize with complete() reported by Paul.) > I ran it for a while and everything seems to work correctly: Tested-by: Sasha Levin <sasha.levin@xxxxxxxxxx> Thanks, Sasha -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
