Re: [PATCH V3 17/23] xen/ioreq: Introduce domain_has_ioreq_server()

[Oleksandr <olekstysh@xxxxxxxxx>]

On 10.12.20 10:38, Paul Durrant wrote:

Hi Paul.

> > -----Original Message-----
> > From: Oleksandr <olekstysh@xxxxxxxxx>
> > Sent: 09 December 2020 20:36
> > To: paul@xxxxxxx
> > Cc: 'Jan Beulich' <jbeulich@xxxxxxxx>; 'Oleksandr Tyshchenko' 
> > <oleksandr_tyshchenko@xxxxxxxx>;
> > 'Stefano Stabellini' <sstabellini@xxxxxxxxxx>; 'Julien Grall' <julien@xxxxxxx>; 
> > 'Volodymyr Babchuk'
> > <Volodymyr_Babchuk@xxxxxxxx>; 'Andrew Cooper' <andrew.cooper3@xxxxxxxxxx>; 
> > 'George Dunlap'
> > <george.dunlap@xxxxxxxxxx>; 'Ian Jackson' <iwj@xxxxxxxxxxxxxx>; 'Wei Liu' 
> > <wl@xxxxxxx>; 'Julien Grall'
> > <julien.grall@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> > Subject: Re: [PATCH V3 17/23] xen/ioreq: Introduce domain_has_ioreq_server()
> >
> >
> > Hi Paul.
> >
> >
> > > > > > > > On 30.11.2020 11:31, Oleksandr Tyshchenko wrote:
> > > > > > > > > --- a/xen/include/xen/ioreq.h
> > > > > > > > > +++ b/xen/include/xen/ioreq.h
> > > > > > > > > @@ -55,6 +55,20 @@ struct ioreq_server {
> > > > > > > > >          uint8_t                bufioreq_handling;
> > > > > > > > >      };
> > > > > > > > >      +/*
> > > > > > > > > + * This should only be used when d == current->domain and it's not
> > > > > > > > > paused,
> > > > > > > > Is the "not paused" part really relevant here? Besides it being rare
> > > > > > > > that the current domain would be paused (if so, it's in the process
> > > > > > > > of having all its vCPU-s scheduled out), does this matter at all?do
> > > > > > > > any extra actionsdo any extra actions
> > > > > > > No, it isn't relevant, I will drop it.
> > > > > > >
> > > > > > >
> > > > > > > > Apart from this the patch looks okay to me, but I'm not sure it
> > > > > > > > addresses Paul's concerns. Iirc he had suggested to switch back to
> > > > > > > > a list if doing a swipe over the entire array is too expensive in
> > > > > > > > this specific case.
> > > > > > > We would like to avoid to do any extra actions in
> > > > > > > leave_hypervisor_to_guest() if possible.
> > > > > > > But not only there, the logic whether we check/set
> > > > > > > mapcache_invalidation variable could be avoided if a domain doesn't
> > > > > > > use IOREQ server...
> > > > > > Are you OK with this patch (common part of it)?
> > > > > How much of a performance benefit is this? The array is small to simply 
> > > > > counting the non-NULL
> > > > entries should be pretty quick.
> > > > I didn't perform performance measurements on how much this call consumes.
> > > > In our system we run three domains. The emulator is in DomD only, so I
> > > > would like to avoid to call vcpu_ioreq_handle_completion() for every
> > > > Dom0/DomU's vCPUs
> > > > if there is no real need to do it.
> > > This is not relevant to the domain that the emulator is running in; it's 
> > > concerning the domains
> > which the emulator is servicing. How many of those are there?
> > Err, yes, I wasn't precise when providing an example.
> > Single emulator is running in DomD and servicing DomU. So with the
> > helper in place the vcpu_ioreq_handle_completion() gets only called for
> > DomU vCPUs (as expected).
> > Without an optimization the vcpu_ioreq_handle_completion() gets called
> > for _all_ vCPUs, and I see it as an extra action for Dom0, DomD vCPUs.
> >
> >
> > > > On Arm vcpu_ioreq_handle_completion()
> > > > is called with IRQ enabled, so the call is accompanied with
> > > > corresponding irq_enable/irq_disable.
> > > > These unneeded actions could be avoided by using this simple one-line
> > > > helper...
> > > The helper may be one line but there is more to the patch than that. I still 
> > > think you could just
> > walk the array in the helper rather than keeping a running occupancy count.
> >
> > OK, is the implementation below close to what you propose? If yes, I
> > will update a helper and drop nr_servers variable.
> >
> > bool domain_has_ioreq_server(const struct domain *d)
> > {
> >       const struct ioreq_server *s;
> >       unsigned int id;
> >
> >       FOR_EACH_IOREQ_SERVER(d, id, s)
> >           return true;
> >
> >       return false;
> > }
> Yes, that's what I had in mind.
>
>    Paul
Thank you for the clarification.


--
Regards,

Oleksandr Tyshchenko