Re: [RFC] Avoid mutex starvation when optimistic spinning is disabled

[Waiman Long]

On 07/21/2016 04:57 PM, Jason Low wrote:
> On Wed, 2016-07-20 at 16:29 +0300, Imre Deak wrote:
> > On ti, 2016-07-19 at 21:39 -0700, Jason Low wrote:
> > > On Tue, 2016-07-19 at 16:04 -0700, Jason Low wrote:
> > > > Hi Imre,
> > > >
> > > > Here is a patch which prevents a thread from spending too much "time"
> > > > waiting for a mutex in the !CONFIG_MUTEX_SPIN_ON_OWNER case.
> > > >
> > > > Would you like to try this out and see if this addresses the mutex
> > > > starvation issue you are seeing in your workload when optimistic
> > > > spinning is disabled?
> > > Although it looks like it didn't take care of the 'lock stealing' case
> > > in the slowpath. Here is the updated fixed version:
> > This also got rid of the problem, I only needed to change the ww
> > functions accordingly. Also, imo mutex_trylock() needs the same
> > handling
> Good point. I supposed mutex_trylock() may not be causing starvation
> issues, but I agree that it makes sense if mutex_trylock() fails too if
> threads are supposed to yield to a waiter. I'll make the update.
>
> Thanks,
> Jason

I think making mutex_trylock() fail maybe a bit too far. Do we really 
have any real workload that cause starvation problem  because of that. 
Code that does mutex_trylock() in a loop can certainly cause lock 
starvation, but it is not how mutex_trylock() is supposed to be used. We 
can't build in safeguard for all the possible abuses of the mutex APIs.

Cheers,
Longman