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

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Message-ID: <ZnBKDRWi_2cO6WbA@macbook>
Date: Mon, 17 Jun 2024 16:37:01 +0200
From: Roger Pau Monné <roger.pau@...rix.com>
To: Jan Beulich <jbeulich@...e.com>
Cc: Frediano Ziglio <frediano.ziglio@...ud.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 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.

> > --- 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.

Thanks, Roger.