RE: [RFC][PATCH 7/9] x86/tsc: Provide sched_clock_noinstr()

Michael Kelley (LINUX)
2023-05-17T02:27:49Z

From: Peter Zijlstra <peterz@infradead.org> Sent: Monday, May 8, 2023 2:44 PM
>
> On Mon, May 08, 2023 at 11:19:58PM +0200, Peter Zijlstra wrote:
>
> > --- a/drivers/clocksource/hyperv_timer.c
> > +++ b/drivers/clocksource/hyperv_timer.c
> > @@ -408,9 +408,9 @@ static u64 notrace read_hv_clock_tsc_cs(
> > return read_hv_clock_tsc();
> > }
> >
> > -static u64 notrace read_hv_sched_clock_tsc(void)
> > +static u64 noinstr read_hv_sched_clock_tsc(void)
> > {
> > - return (read_hv_clock_tsc() - hv_sched_clock_offset) *
> > + return (hv_read_tsc_page(hv_get_tsc_page()) - hv_sched_clock_offset) *
> > (NSEC_PER_SEC / HV_CLOCK_HZ);
> > }
> >
> > --- a/include/clocksource/hyperv_timer.h
> > +++ b/include/clocksource/hyperv_timer.h
> > @@ -38,7 +38,7 @@ extern void hv_remap_tsc_clocksource(voi
> > extern unsigned long hv_get_tsc_pfn(void);
> > extern struct ms_hyperv_tsc_page *hv_get_tsc_page(void);
> >
> > -static inline notrace u64
> > +static __always_inline notrace u64
> > hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg, u64 *cur_tsc)
> > {
> > u64 scale, offset;
> > @@ -85,7 +85,7 @@ hv_read_tsc_page_tsc(const struct ms_hyp
> > return mul_u64_u64_shr(*cur_tsc, scale, 64) + offset;
> > }
> >
> > -static inline notrace u64
> > +static __always_inline notrace u64
> > hv_read_tsc_page(const struct ms_hyperv_tsc_page *tsc_pg)
> > {
> > u64 cur_tsc;
>
> Hyper-V folks!
>
> While reviewing all this I found the following 'gem':
>
> hv_init_clocksource()
> hv_setup_sched_clock()
> paravirt_set_sched_clock(read_hv_sched_clock_msr)
>
> read_hv_sched_clock_msr() [notrace]
> read_hv_clock_msr() [notrace]
> hv_get_register() *traced*
> hv_get_non_nested_register() ...
> hv_ghcb_msr_read()
> WARN_ON(in_nmi())
> ...
> local_irq_save()
>
>
> Note that:
>
> a) sched_clock() is used in NMI context a *LOT*
> b) sched_clock() is notrace (or even noinstr with these patches)
> and local_irq_save() implies tracing
>
>
> Can you pretty please:
>
> 1) delete all this; or,
> 2) fix it in a hurry?
>

Peter -- I've sent you an RFC patch to incorporate into your broader
patch set. I think it probably makes sense for all the Hyper-V
stuff to be a separate patch.

I haven't previously worked with the details of notrace vs. noinstr,
but I followed the patterns elsewhere in patch set. Please review
to see if it seems correct.

One thing: In the cases where I added __always_inline, I dropped
any notrace or noinstr annotations. I presume such code always
takes on the attributes of the caller. If that's not correct, let me know.

Michael