linux-kernel - Re: [PATCH] x86/xen/time: Reduce Xen timer tick

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Date: Tue, 18 Jun 2024 09:37:08 +0100
From: Frediano Ziglio <frediano.ziglio@...ud.com>
To: Roger Pau Monné <roger.pau@...rix.com>
Cc: Jan Beulich <jbeulich@...e.com>, "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Borislav Petkov <bp@...en8.de>, 
	Ingo Molnar <mingo@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Boris Ostrovsky <boris.ostrovsky@...cle.com>, Juergen Gross <jgross@...e.com>, 
	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/xen/time: Reduce Xen timer tick

On Mon, Jun 17, 2024 at 3:37 PM Roger Pau Monné <roger.pau@...rix.com> wrote:
>
> On Mon, Jun 17, 2024 at 04:22:21PM +0200, Jan Beulich wrote:
> > On 17.06.2024 16:13, Frediano Ziglio wrote:
> > > Current timer tick is causing some deadline to fail.
> > > The current high value constant was probably due to an old
> > > bug in the Xen timer implementation causing errors if the
> > > deadline was in the future.
> > > This was fixed in Xen commit:
> > > 19c6cbd90965 xen/vcpu: ignore VCPU_SSHOTTMR_future
> >
> > And then newer kernels are no longer reliably usable on Xen older than
> > this?
>
> I think this should reference the Linux commit that removed the usage
> of VCPU_SSHOTTMR_future on Linux itself, not the change that makes Xen
> ignore the flag.
>

Yes, Linux kernel stopped using this flag since 2016 with commit
c06b6d70feb32d28f04ba37aa3df17973fd37b6b, "xen/x86: don't lose event
interrupts", I'll add it in the commit message.

> > > --- a/arch/x86/xen/time.c
> > > +++ b/arch/x86/xen/time.c
> > > @@ -30,7 +30,7 @@
> > >  #include "xen-ops.h"
> > >
> > >  /* Minimum amount of time until next clock event fires */
> > > -#define TIMER_SLOP 100000
> > > +#define TIMER_SLOP 1000
> >
> > It may be just the lack of knowledge of mine towards noadays's Linux'es
> > time handling, but the change of a value with this name and thus
> > commented doesn't directly relate to "timer tick" rate. Could you maybe
> > help me see the connection?
>
> The TIMER_SLOP define is used in min_delta_{ns,ticks} field, and I
> think this is wrong.
>
> The min_delta_ns for the Xen timer is 1ns.  If Linux needs some
> greater min delta than what the timer interface supports it should be
> handled in the generic timer code, not open coded at the definition of
> possibly each timer implementation.
>

I think this is done to reduce potential event handling frequency, in
some other part of timer code (in kernel/time/clockevents.c) there's a
comment "Deltas less than 1usec are pointless noise".
I think it's hard for a software to get a frequency so high so I
didn't propose 1ns.
What are you suggesting? To put 1ns and see what happens? Is there any
proper test code for this?

> Thanks, Roger.

Frediano