lkml.org 
[lkml]   [2015]   [Jul]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 1/7] locking/pvqspinlock: Only kick CPU at unlock time
On 07/13/2015 08:02 AM, Peter Zijlstra wrote:
> On Sat, Jul 11, 2015 at 04:36:52PM -0400, Waiman Long wrote:
>> @@ -181,9 +187,9 @@ static void pv_wait_node(struct mcs_spinlock *node)
>> pv_wait(&pn->state, vcpu_halted);
>>
>> /*
>> - * Reset the vCPU state to avoid unncessary CPU kicking
>> + * Reset the state except when vcpu_hashed is set.
>> */
>> - WRITE_ONCE(pn->state, vcpu_running);
>> + cmpxchg(&pn->state, vcpu_halted, vcpu_running);
> Why? Suppose we did get advanced into the hashed state, and then get a
> (spurious) wakeup, this means we'll observe our ->locked == 1 condition
> and fall out of pv_wait_node().
>
> We'll then enter pv_wait_head(), which with your modification:
>
>> @@ -229,19 +244,42 @@ static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
>> {
>> struct pv_node *pn = (struct pv_node *)node;
>> struct __qspinlock *l = (void *)lock;
>> - struct qspinlock **lp = NULL;
>> + struct qspinlock **lp;
>> int loop;
>>
>> + /*
>> + * Initialize lp to a non-NULL value if it has already been in the
>> + * pv_hashed state so that pv_hash() won't be called again.
>> + */
>> + lp = (READ_ONCE(pn->state) == vcpu_hashed) ? (struct qspinlock **)1
>> + : NULL;
>> for (;;) {
>> + WRITE_ONCE(pn->state, vcpu_running);
> Will instantly and unconditionally write vcpu_running.
>
>

This code is kind of complicated. I am going to get rid of the current
tri-state setup, and switch to a separate sync variable for defer kicking.

Cheers,
Longman


\
 
 \ /
  Last update: 2015-07-15 03:41    [W:0.146 / U:0.136 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site