Re: [Xen-devel] [PATCH] xen/sched: rework credit2 run-queue allocation

[Dario Faggioli <dfaggioli@xxxxxxxx>]

On Wed, 2020-02-19 at 17:52 +0100, Jan Beulich wrote:
> On 19.02.2020 17:47, Dario Faggioli wrote:
> > On Thu, 2020-01-23 at 09:55 +0100, Juergen Gross wrote:
> > > --- a/xen/common/sched/credit2.c
> > > +++ b/xen/common/sched/credit2.c
> > > @@ -849,51 +822,71 @@ static inline bool same_core(unsigned int
> > > cpua,
> > > unsigned int cpub)
> > >             cpu_to_core(cpua) == cpu_to_core(cpub);
> > >  }
> > >  
> > > -static unsigned int
> > > -cpu_to_runqueue(const struct csched2_private *prv, unsigned int
> > > cpu)
> > > +static struct csched2_runqueue_data *
> > > +cpu_add_to_runqueue(struct csched2_private *prv, unsigned int
> > > cpu)
> > >  {
> > > -    const struct csched2_runqueue_data *rqd;
> > > -    unsigned int rqi;
> > > +    struct csched2_runqueue_data *rqd, *rqd_new;
> > > +    struct list_head *rqd_ins;
> > > +    unsigned long flags;
> > > +    int rqi = 0;
> > > +    bool rqi_unused = false, rqd_valid = false;
> > > +
> > > +    rqd_new = xzalloc(struct csched2_runqueue_data);
> > >  
> > > 
> > So, I'm not sure I see why it's better to allocating this here, and
> > then free it if we didn't need it, instead than allocating it
> > later,
> > only if we actually need it... What am I missing? :-)
> 
> Where possible we should try to avoid calling allocation functions
> with locks held.
> 
Ah, sure, that's a very good reason indeed, my bad not noticing that
doing this like that the allocation sits outside of the write_lock()
section.

Nevertheless, I'd add a quick comment about that, to make it even more
obvious. :-)

Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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<<This happens because _I_ choose it to happen!>> (Raistlin Majere)

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