Messages in this thread | | | From | "Chen, Dennis (SRDC SW)" <> | Subject | RE: semaphore and mutex in current Linux kernel (3.2.2) | Date | Wed, 11 Apr 2012 05:04:03 +0000 |
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On Tue, Apr 10, 2012 at 2:45 AM, Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote: > On Fri, Apr 06, 2012 at 05:47:28PM +0000, Chen, Dennis (SRDC SW) wrote: >> On Fri, Apr 6, 2012 at 6:10 PM, Clemens Ladisch <clemens@ladisch.de> wrote: >> > Chen, Dennis (SRDC SW) wrote: >> > >> > "On the internet, nobody can hear you being subtle." >> > >> > If some other process wants to run on the same CPU, needs_resched() is set. >> > (This might happen to make the cursor blink, for keyboard input, or when >> > somebody starts a rogue process like ps.) >> > >> >> Hmm, I forget that in each timer interrupt, __rcu_pending() will be called, it will call >> set_need_resched() to set the TIF_NEED_RESCHED in some condition... >> The optimization of mutex work closely with rcu, so fantastic! > > I must confess that you all lost me on this one. > > There is a call to set_need_resched() in __rcu_pending(), which is > invoked when the current CPU has not yet responded to a non-preemptible > RCU grace period for some time. However, in the common case where the > CPUs all respond in reasonable time, __rcu_pending() will never call > set_need_resched(). > > However, we really do not want to call set_need_resched() on every call > to __rcu_pending(). There is almost certainly a better solution to any > problem that might be solved by a per-jiffy call to set_need_resched(). > > So, what are you really trying to do? > > Thanx, Paul
Paul, I must confess that maybe you're right, I've realized the misunderstanding in the previous email. But I don't want to pretend that I have a full understanding for your " There is almost certainly a better solution to any problem that might be solved by a per-jiffy call to set_need_resched()", because this is related with your last question.
I just want to measure the performance between semaphore and mutex, before that I looked at the mutex optimization code, and the focus is on the mutex_spin_on_owner() function, I don't know how long it will take before some components in the kernel call set_need_resched() to break the while loop. If it's on jiffies level, given the time of a process switch possibly in microsecond level, that means current process must spin for several jiffies before it got the mutex lock or go to sleep finally, I can't see the benefit here...
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