在 16/7/15 16:47, Peter Zijlstra 写道:
So the reason I never get around to this is because the patch stinks.
It simply doesn't make sense... Remember, the harder you make a reviewer
work the less likely the review will be done.
Present things in clear concise language and draw a picture.
On Tue, May 31, 2016 at 12:53:48PM -0400, Waiman Long wrote:
Currently, calling pv_hash() and setting _Q_SLOW_VAL is only
done once for any pv_node. It is either in pv_kick_node() or in
pv_wait_head_or_lock().
So far so good....
Because of lock stealing, a pv_kick'ed node is
not guaranteed to get the lock before the spinning threshold expires
and has to call pv_wait() again. As a result, the new lock holder
won't see _Q_SLOW_VAL and so won't wake up the sleeping vCPU.
waiman, it might be "as a result, the head node will not really enter wait state
because ->locked is not_Q_SLOW_VAL, the pv_wait will return directly."
*brain melts* what!? pv_kick'ed node reads like pv_kick_node() and that
doesn't make any kind of sense.
I'm thinking you're trying to say this:
CPU0 CPU1 CPU2
__pv_queued_spin_unlock_slowpath()
...
smp_store_release(&l->locked, 0);
__pv_queued_spin_lock_slowpath()
...
pv_queued_spin_steal_lock()
cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0
pv_wait_head_or_lock()
pv_kick(node->cpu); ----------------------> pv_wait(&l->locked,
_Q_SLOW_VAL);
__pv_queued_spin_unlock()
cmpxchg(&l->locked, _Q_LOCKED_VAL, 0) == _Q_LOCKED_VAL
for () {
trylock_clear_pending();
cpu_relax();
}
pv_wait(&l->locked,
_Q_SLOW_VAL);
Which is indeed 'bad', but not fatal, note that the later pv_wait() will
not in fact go wait, since l->locked will _not_ be _Q_SLOW_VAL.
hi, Peter
the problem is that "this later pv_wait will do nothing as l->locked is not
_Q_SLOW_VAL",
So it is not paravirt friendly then. we will go into the trylock loop again and
again until the lock is unlocked.
So if we are kicked by the unlock_slowpath, and the lock is stealed by someone
else, we need hash its node again and set l->locked to _Q_SLOW_VAL, then enter
pv_wait.
but I am worried about lock stealing. could the node in the queue starve for a
long time? I notice the latency of pv_wait on an over-commited guest can be
bigger than 300us. I have not seen such starving case, but I think it is
possible to happen.
thanks
xinhui
Is this indeed the 3 CPU scenario you tried to describe in a scant 4
lines of text, or is there more to it?
