lkml.org 
[lkml]   [2014]   [Jul]   [3]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RFC] Cancellable MCS spinlock rework
From
Date
On Thu, 2014-07-03 at 16:35 -0400, Waiman Long wrote:

> I do see a point in reducing the size of the rwsem structure. However, I
> don't quite understand the point of converting pointers in the
> optimistic_spin_queue structure to atomic_t. The structure is cacheline
> aligned and there is no saving in size. Converting them to atomic_t does
> have a bit of additional overhead of converting the encoded cpu number
> back to the actual pointer.
>
> So my suggestion is to just change what is stored in the mutex and rwsem
> structure to atomic_t, but keep the pointers in the
> optimistic_spin_queue structure.

Peter, would you prefer going with the above?

If we were to keep the pointers to the next and prev nodes in the struct
optimistic_spin_queue instead of converting them to atomic_t to store
their cpu #, we'd still need to keep track of the cpu #. In the unqueue
phase of osq_lock, we might have to reload prev = node->prev which we
then may cmpxchg() it with the lock tail.

The method we can think of so far would be to add a regular int variable
to optimistic_spin_queue and initialize it to the CPU #, during the time
we also initialize node->locked and node->next at the beginning of
osq_lock. The cost wouldn't be much of an issue since
optimistic_spin_queue is cache aligned.



\
 
 \ /
  Last update: 2014-07-04 03:41    [W:0.101 / U:1.864 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site