On Tue, Feb 24, 2015 at 02:54:59PM +0530, 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> > Reviewed-by: Oleg Nesterov <oleg@xxxxxxxxxx> > Acked-by: David Vrabel <david.vrabel@xxxxxxxxxx> > --- > arch/x86/include/asm/spinlock.h | 94 ++++++++++++++++++++--------------------- > arch/x86/kernel/kvm.c | 7 ++- > arch/x86/xen/spinlock.c | 7 ++- > 3 files changed, 58 insertions(+), 50 deletions(-) > > Changes for stable: > - Don't replace the ACCESS_ONCE to READ_ONCE which would cause horraneous > Compiler warnings (Linus, David Vbriel, PeterZ, Ingo) What is the git commit id of this in Linus's tree? What stable tree(s) do you want this applied to? thanks, greg k-h -- 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
