RE: [PATCH 05/10] viridian: use softirq batching in hvcall_ipi()

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: 12 November 2020 08:53
> To: Paul Durrant <paul@xxxxxxx>
> Cc: Paul Durrant <pdurrant@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>; Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx>; Roger Pau Monné <roger.pau@xxxxxxxxxx>; 
> xen-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH 05/10] viridian: use softirq batching in hvcall_ipi()
> 
> On 11.11.2020 21:07, Paul Durrant wrote:
> > From: Paul Durrant <pdurrant@xxxxxxxxxx>
> >
> > vlapic_ipi() uses a softirq batching mechanism to improve the efficiency of
> > sending a IPIs to large number of processors. This patch modifies send_ipi()
> > (the worker function called by hvcall_ipi()) to also make use of the
> > mechanism when there multiple bits set the hypercall_vpmask. Hence a `nr`
> > field is added to the structure to track the number of set bits.
> 
> This is kind of unusual, i.e. we don't do so elsewhere. I take it the
> assumption is that using bitmap_weight() is too much overhead?

It just seemed wasteful in the circumstances. If I move to bitmap copy OTOH 
then I'll have to use bitmap_weight().

> 
> > @@ -509,6 +510,7 @@ void viridian_domain_deinit(struct domain *d)
> >
> >  struct hypercall_vpmask {
> >      DECLARE_BITMAP(mask, HVM_MAX_VCPUS);
> > +    unsigned int nr;
> >  };
> >
> >  static DEFINE_PER_CPU(struct hypercall_vpmask, hypercall_vpmask);
> > @@ -516,21 +518,24 @@ static DEFINE_PER_CPU(struct hypercall_vpmask, 
> > hypercall_vpmask);
> >  static void vpmask_empty(struct hypercall_vpmask *vpmask)
> >  {
> >      bitmap_zero(vpmask->mask, HVM_MAX_VCPUS);
> > +    vpmask->nr = 0;
> >  }
> >
> >  static void vpmask_set(struct hypercall_vpmask *vpmask, unsigned int vp)
> >  {
> > -    __set_bit(vp, vpmask->mask);
> > +    if ( !test_and_set_bit(vp, vpmask->mask) )
> > +        vpmask->nr++;
> 
> If test_and_set_bit() is the correct thing to use here (rather
> than __test_and_set_bit()), the counter also needs updating
> atomically.
> 

It doesn't need to be atomic, but I'll probably drop it.

> >  }
> >
> >  static void vpmask_fill(struct hypercall_vpmask *vpmask)
> >  {
> >      bitmap_fill(vpmask->mask, HVM_MAX_VCPUS);
> > +    vpmask->nr = HVM_MAX_VCPUS;
> >  }
> >
> >  static bool vpmask_test(struct hypercall_vpmask *vpmask, unsigned int vp)
> >  {
> > -    return test_bit(vp, vpmask->mask);
> > +    return vpmask->nr && test_bit(vp, vpmask->mask);
> 
> Is this in fact an improvement?
> 

I think so, but it's a moot point if I drop 'nr'.

  Paul

> Jan