Re: [Xen-devel] [PATCH v6 16/18] vmx: Add some scheduler hooks for VT-d posted interrupts

["Wu, Feng" <feng.wu@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Thursday, September 10, 2015 9:15 PM
> To: Wu, Feng
> Cc: Andrew Cooper; Dario Faggioli; George Dunlap; Tian, Kevin;
> xen-devel@xxxxxxxxxxxxx; Keir Fraser
> Subject: RE: [PATCH v6 16/18] vmx: Add some scheduler hooks for VT-d posted
> interrupts
> 
> >>> On 10.09.15 at 14:58, <feng.wu@xxxxxxxxx> wrote:
> 
> >
> >> -----Original Message-----
> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> Sent: Thursday, September 10, 2015 8:45 PM
> >> To: Wu, Feng
> >> Cc: Andrew Cooper; Dario Faggioli; George Dunlap; Tian, Kevin;
> >> xen-devel@xxxxxxxxxxxxx; Keir Fraser
> >> Subject: RE: [PATCH v6 16/18] vmx: Add some scheduler hooks for VT-d
> posted
> >> interrupts
> >>
> >> >>> On 10.09.15 at 14:34, <feng.wu@xxxxxxxxx> wrote:
> >>
> >> >
> >> >> -----Original Message-----
> >> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> >> Sent: Thursday, September 10, 2015 6:01 PM
> >> >> To: Wu, Feng
> >> >> Cc: Andrew Cooper; Dario Faggioli; George Dunlap; Tian, Kevin;
> >> >> xen-devel@xxxxxxxxxxxxx; Keir Fraser
> >> >> Subject: RE: [PATCH v6 16/18] vmx: Add some scheduler hooks for VT-d
> >> posted
> >> >> interrupts
> >> >>
> >> >> >>> On 10.09.15 at 11:41, <feng.wu@xxxxxxxxx> wrote:
> >> >> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> >> >> Sent: Thursday, September 10, 2015 5:26 PM
> >> >> >> >>> On 10.09.15 at 10:59, <feng.wu@xxxxxxxxx> wrote:
> >> >> >> > First, how to check it while waiting to acquire the lock 
> >> >> >> > .pi_block_cpu
> >> >> >> > didn't change?
> >> >> >>
> >> >> >> Note the difference between "check while waiting" and "check that
> >> >> >> while waiting": The former is indeed hard to implement, while the
> >> >> >> latter is pretty straightforward (and we do so elsewhere).
> >> >> >>
> >> >> >> > Secondly, even if we can check it, what should we do if 
> >> >> >> > .pi_block_cpu
> >> >> >> > is changed after acquiring the lock as I mentioned above?
> >> >> >>
> >> >> >> Drop the lock and start over. I.e. (taking your pseudo code)
> >> >> >>
> >> >> >> restart:
> >> >> >>     local_pi_block_cpu = ...;
> >> >> >>     bail-if-invalid (e.g. -1 in current model)
> >> >> >>     spin_lock_irqsave(&per_cpu(, local_pi_block_cpu), flags);
> >> >> >>     if(local_pi_block_cpu != actual_pi_block_cpu) {
> >> >> >>         spin_unlock_irqrestore(&per_cpu(,local_pi_block_cpu),
> flags);
> >> >> >>         goto restart;
> >> >> >>     }
> >> >> >
> >> >> > Thanks a lot for showing me this pseudo code! My concern is if
> >> >> > .pi_block_vcpu is changed to -1 at this point, it doesn't work.
> >> >> > .pi_block_vcpu being -1 here means the vCPU is remove from
> >> >> > the blocking list by others, then we cannot delete it again via
> >> >> > list_del() here.
> >> >>
> >> >> Did you miss the "bail-if-invalid" above?
> >> >
> >> > I am sorry, do I miss something here? If .pi_block_cpu becomes
> >> > -1 here (after the above 'if' statement is finished with
> >> > local_pi_block_cpu == actual_pi_block_cpu ), how can "bail-if-invalid"
> >> > above help?
> >>
> >> The (obvious I thought) implication is that all assignments to
> >> pi_block_cpu (along with all list manipulations) now need to happen
> >> with the lock held.
> >
> > If all the assignment to pi_block_cpu is with one lock held, I don't think
> > we need to above checkout, we can safely use .pi_block_cpu, right?
> 
> No. In order to use it you need to make sure it's valid (or else
> there's no valid lock to acquire). And once you acquired the
> lock, you have to check whether changed / became invalid.

If all the assignment to .pi_block_cpu is with one lock held,
how can it get changed / become invalid then once I acquired
the lock?

Thanks,
Feng

> Plus the up front check allows to avoid acquiring the lock in the
> first place when the field is invalid anyway.
> 
> Jan


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