Re: [PATCH 08/11] x86/tsc: Avoid synchronizing TSCs with multiple CPUs in parallel

[David Woodhouse]

On Thu, 2021-12-09 at 16:43 +0100, Peter Zijlstra wrote:
> On Thu, Dec 09, 2021 at 03:09:35PM +0000, David Woodhouse wrote:
> > diff --git a/arch/x86/kernel/tsc_sync.c b/arch/x86/kernel/tsc_sync.c
> > index 50a4515fe0ad..4ee247d89a49 100644
> > --- a/arch/x86/kernel/tsc_sync.c
> > +++ b/arch/x86/kernel/tsc_sync.c
> > @@ -202,6 +202,7 @@ bool tsc_store_and_check_tsc_adjust(bool bootcpu)
> >   * Entry/exit counters that make sure that both CPUs
> >   * run the measurement code at once:
> >   */
> > +static atomic_t tsc_sync_cpu = ATOMIC_INIT(-1);
> >  static atomic_t start_count;
> >  static atomic_t stop_count;
> >  static atomic_t skip_test;
> > @@ -326,6 +327,8 @@ void check_tsc_sync_source(int cpu)
> >  		atomic_set(&test_runs, 1);
> >  	else
> >  		atomic_set(&test_runs, 3);
> > +
> > +	atomic_set(&tsc_sync_cpu, cpu);
> >  retry:
> >  	/*
> >  	 * Wait for the target to start or to skip the test:
> > @@ -407,6 +410,10 @@ void check_tsc_sync_target(void)
> >  	if (unsynchronized_tsc())
> >  		return;
> >  
> > +	/* Wait for this CPU's turn */
> > +	while (atomic_read(&tsc_sync_cpu) != cpu)
> > +		cpu_relax();
> > +
> >  	/*
> >  	 * Store, verify and sanitize the TSC adjust register. If
> >  	 * successful skip the test.
> 
> This new atomic_t seems superfluous, there isn't any actual atomic
> operation used.

That's true; it could just be WRITE_ONCE/READ_ONCE. But the atomic is
fairly much equivalent and does no harm.

I see this one mostly as a placeholder — I'd still prefer to have a
decent 1:many TSC sync or at least a 1:many *check* falling back to 1:1
mode if anything actually needs to be adjusted. And/or just avoid the
TSC sync completely when it's not needed, like on kexec.

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