Re: [PATCH 1/2] x86/tsc: Add new BPF helper call bpf_rdtsc

[John Fastabend]

Alexei Starovoitov wrote:
> On Mon, Jul 3, 2023 at 3:58 AM Tero Kristo <tero.kristo@xxxxxxxxxxxxxxx> wrote:
> >
> > Currently the raw TSC counter can be read within kernel via rdtsc_ordered()
> > and friends, and additionally even userspace has access to it via the
> > RDTSC assembly instruction. BPF programs on the other hand don't have
> > direct access to the TSC counter, but alternatively must go through the
> > performance subsystem (bpf_perf_event_read), which only provides relative
> > value compared to the start point of the program, and is also much slower
> > than the direct read. Add a new BPF helper definition for bpf_rdtsc() which
> > can be used for any accurate profiling needs.
> >
> > A use-case for the new API is for example wakeup latency tracing via
> > eBPF on Intel architecture, where it is extremely beneficial to be able
> > to get raw TSC timestamps and compare these directly to the value
> > programmed to the MSR_IA32_TSC_DEADLINE register. This way a direct
> > latency value from the hardware interrupt to the execution of the
> > interrupt handler can be calculated. Having the functionality within
> > eBPF also has added benefits of allowing to filter any other relevant
> > data like C-state residency values, and also to drop any irrelevant
> > data points directly in the kernel context, without passing all the
> > data to userspace for post-processing.
> >
> > Signed-off-by: Tero Kristo <tero.kristo@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/include/asm/msr.h |  1 +
> >  arch/x86/kernel/tsc.c      | 23 +++++++++++++++++++++++
> >  2 files changed, 24 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
> > index 65ec1965cd28..3dde673cb563 100644
> > --- a/arch/x86/include/asm/msr.h
> > +++ b/arch/x86/include/asm/msr.h
> > @@ -309,6 +309,7 @@ struct msr *msrs_alloc(void);
> >  void msrs_free(struct msr *msrs);
> >  int msr_set_bit(u32 msr, u8 bit);
> >  int msr_clear_bit(u32 msr, u8 bit);
> > +u64 bpf_rdtsc(void);
> >
> >  #ifdef CONFIG_SMP
> >  int rdmsr_on_cpu(unsigned int cpu, u32 msr_no, u32 *l, u32 *h);
> > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> > index 344698852146..ded857abef81 100644
> > --- a/arch/x86/kernel/tsc.c
> > +++ b/arch/x86/kernel/tsc.c
> > @@ -15,6 +15,8 @@
> >  #include <linux/timex.h>
> >  #include <linux/static_key.h>
> >  #include <linux/static_call.h>
> > +#include <linux/btf.h>
> > +#include <linux/btf_ids.h>
> >
> >  #include <asm/hpet.h>
> >  #include <asm/timer.h>
> > @@ -29,6 +31,7 @@
> >  #include <asm/intel-family.h>
> >  #include <asm/i8259.h>
> >  #include <asm/uv/uv.h>
> > +#include <asm/tlbflush.h>
> >
> >  unsigned int __read_mostly cpu_khz;    /* TSC clocks / usec, not used here */
> >  EXPORT_SYMBOL(cpu_khz);
> > @@ -1551,6 +1554,24 @@ void __init tsc_early_init(void)
> >         tsc_enable_sched_clock();
> >  }
> >
> > +u64 bpf_rdtsc(void)
> > +{
> > +       /* Check if Time Stamp is enabled only in ring 0 */
> > +       if (cr4_read_shadow() & X86_CR4_TSD)
> > +               return 0;
> 
> Why check this? It's always enabled in the kernel, no?
> 
> > +
> > +       return rdtsc_ordered();
> 
> Why _ordered? Why not just rdtsc ?
> Especially since you want to trace latency. Extra lfence will ruin
> the measurements.
> 

If we used it as a fast way to order events on multiple CPUs I
guess we need the lfence? We use ktime_get_ns() now for things
like this when we just need an order counter. We have also
observed time going backwards with this and have heuristics
to correct it but its rare.