Messages in this thread Patch in this message | | | Date | Mon, 3 Jul 2023 21:49:41 -0700 | Subject | Re: [PATCH 1/2] x86/tsc: Add new BPF helper call bpf_rdtsc | From | Yonghong Song <> |
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On 7/3/23 3:57 AM, Tero Kristo 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@linux.intel.com> > --- > 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)
Please see kernel/bpf/helpers.c. For kfunc definition, we should have
__diag_push(); __diag_ignore_all("-Wmissing-prototypes", "Global functions as their definitions will be in vmlinux BTF");
_bpf_kfunc u64 bpf_rdtsc(void) { ... }
__diag_pop();
> +{ > + /* Check if Time Stamp is enabled only in ring 0 */ > + if (cr4_read_shadow() & X86_CR4_TSD) > + return 0; > + > + return rdtsc_ordered(); > +} > + > +BTF_SET8_START(tsc_bpf_kfunc_ids) > +BTF_ID_FLAGS(func, bpf_rdtsc) > +BTF_SET8_END(tsc_bpf_kfunc_ids) > + > +static const struct btf_kfunc_id_set tsc_bpf_kfunc_set = { > + .owner = THIS_MODULE, > + .set = &tsc_bpf_kfunc_ids, > +}; > + > void __init tsc_init(void) > { > if (!cpu_feature_enabled(X86_FEATURE_TSC)) { > @@ -1594,6 +1615,8 @@ void __init tsc_init(void) > > clocksource_register_khz(&clocksource_tsc_early, tsc_khz); > detect_art(); > + > + register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &tsc_bpf_kfunc_set);
register_btf_kfunc_id_set() could fail, maybe you at least wants to have a warning so bpf prog users may be aware that kfunc bpf_rdtsc() not really available to bpf programs?
> } > > #ifdef CONFIG_SMP
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