Re: [PATCH v2 05/10] riscv: Prepare for user-space perf event mmap support

Alexandre Ghiti
2023-06-16T08:57:42Z

On Fri, Jun 16, 2023 at 10:28 AM Atish Patra <atishp@atishpatra.org> wrote:
>
> On Fri, May 12, 2023 at 1:58 AM Alexandre Ghiti <alexghiti@rivosinc.com> wrote:
> >
> > Provide all the necessary bits in the generic riscv pmu driver to be
> > able to mmap perf events in userspace: the heavy lifting lies in the
> > driver backend, namely the legacy and sbi implementations.
> >
> > Note that arch_perf_update_userpage is almost a copy of arm64 code.
> >
> > Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> > ---
> > arch/riscv/kernel/Makefile | 2 +-
> > arch/riscv/kernel/perf_event.c | 58 ++++++++++++++++++++++++++++++++++
> > drivers/perf/riscv_pmu.c | 41 ++++++++++++++++++++++++
> > include/linux/perf/riscv_pmu.h | 4 +++
> > 4 files changed, 104 insertions(+), 1 deletion(-)
> > create mode 100644 arch/riscv/kernel/perf_event.c
> >
> > diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> > index 4cf303a779ab..0d215fd9860d 100644
> > --- a/arch/riscv/kernel/Makefile
> > +++ b/arch/riscv/kernel/Makefile
> > @@ -70,7 +70,7 @@ obj-$(CONFIG_DYNAMIC_FTRACE) += mcount-dyn.o
> >
> > obj-$(CONFIG_TRACE_IRQFLAGS) += trace_irq.o
> >
> > -obj-$(CONFIG_PERF_EVENTS) += perf_callchain.o
> > +obj-$(CONFIG_PERF_EVENTS) += perf_callchain.o perf_event.o
> > obj-$(CONFIG_HAVE_PERF_REGS) += perf_regs.o
> > obj-$(CONFIG_RISCV_SBI) += sbi.o
> > ifeq ($(CONFIG_RISCV_SBI), y)
> > diff --git a/arch/riscv/kernel/perf_event.c b/arch/riscv/kernel/perf_event.c
> > new file mode 100644
> > index 000000000000..94174a0fc251
> > --- /dev/null
> > +++ b/arch/riscv/kernel/perf_event.c
> > @@ -0,0 +1,58 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +#include <linux/sched_clock.h>
> > +
> > +void arch_perf_update_userpage(struct perf_event *event,
> > + struct perf_event_mmap_page *userpg, u64 now)
> > +{
> > + struct clock_read_data *rd;
> > + unsigned int seq;
> > + u64 ns;
> > +
> > + userpg->cap_user_time = 0;
> > + userpg->cap_user_time_zero = 0;
> > + userpg->cap_user_time_short = 0;
> > + userpg->cap_user_rdpmc =
> > + !!(event->hw.flags & PERF_EVENT_FLAG_USER_READ_CNT);
> > +
> > + userpg->pmc_width = 64;
> > +
>
> The counter width is 64 for cycle & instret. Other hpmcounter can have
> different width.
> This information should retrieved from counter info.

Yes, this is done in patch 7 when I adapt the perf SBI backend to
allow the user access.

>
> > + do {
> > + rd = sched_clock_read_begin(&seq);
> > +
> > + userpg->time_mult = rd->mult;
> > + userpg->time_shift = rd->shift;
> > + userpg->time_zero = rd->epoch_ns;
> > + userpg->time_cycles = rd->epoch_cyc;
> > + userpg->time_mask = rd->sched_clock_mask;
> > +
> > + /*
> > + * Subtract the cycle base, such that software that
> > + * doesn't know about cap_user_time_short still 'works'
> > + * assuming no wraps.
> > + */
> > + ns = mul_u64_u32_shr(rd->epoch_cyc, rd->mult, rd->shift);
> > + userpg->time_zero -= ns;
> > +
> > + } while (sched_clock_read_retry(seq));
> > +
> > + userpg->time_offset = userpg->time_zero - now;
> > +
> > + /*
> > + * time_shift is not expected to be greater than 31 due to
> > + * the original published conversion algorithm shifting a
> > + * 32-bit value (now specifies a 64-bit value) - refer
> > + * perf_event_mmap_page documentation in perf_event.h.
> > + */
> > + if (userpg->time_shift == 32) {
> > + userpg->time_shift = 31;
> > + userpg->time_mult >>= 1;
> > + }
> > +
> > + /*
> > + * Internal timekeeping for enabled/running/stopped times
> > + * is always computed with the sched_clock.
> > + */
> > + userpg->cap_user_time = 1;
> > + userpg->cap_user_time_zero = 1;
> > + userpg->cap_user_time_short = 1;
> > +}
> > diff --git a/drivers/perf/riscv_pmu.c b/drivers/perf/riscv_pmu.c
> > index ebca5eab9c9b..af69da268246 100644
> > --- a/drivers/perf/riscv_pmu.c
> > +++ b/drivers/perf/riscv_pmu.c
> > @@ -171,6 +171,8 @@ int riscv_pmu_event_set_period(struct perf_event *event)
> >
> > local64_set(&hwc->prev_count, (u64)-left);
> >
> > + perf_event_update_userpage(event);
> > +
> > return overflow;
> > }
> >
> > @@ -267,6 +269,9 @@ static int riscv_pmu_event_init(struct perf_event *event)
> > hwc->idx = -1;
> > hwc->event_base = mapped_event;
> >
> > + if (rvpmu->event_init)
> > + rvpmu->event_init(event);
> > +
> > if (!is_sampling_event(event)) {
> > /*
> > * For non-sampling runs, limit the sample_period to half
> > @@ -283,6 +288,39 @@ static int riscv_pmu_event_init(struct perf_event *event)
> > return 0;
> > }
> >
> > +static int riscv_pmu_event_idx(struct perf_event *event)
> > +{
> > + struct riscv_pmu *rvpmu = to_riscv_pmu(event->pmu);
> > +
> > + if (!(event->hw.flags & PERF_EVENT_FLAG_USER_READ_CNT))
> > + return 0;
> > +
> > + if (rvpmu->csr_index)
> > + return rvpmu->csr_index(event) + 1;
> > +
> > + return 0;
> > +}
> > +
> > +static void riscv_pmu_event_mapped(struct perf_event *event, struct mm_struct *mm)
> > +{
> > + struct riscv_pmu *rvpmu = to_riscv_pmu(event->pmu);
> > +
> > + if (rvpmu->event_mapped) {
> > + rvpmu->event_mapped(event, mm);
> > + perf_event_update_userpage(event);
> > + }
> > +}
> > +
> > +static void riscv_pmu_event_unmapped(struct perf_event *event, struct mm_struct *mm)
> > +{
> > + struct riscv_pmu *rvpmu = to_riscv_pmu(event->pmu);
> > +
> > + if (rvpmu->event_unmapped) {
> > + rvpmu->event_unmapped(event, mm);
> > + perf_event_update_userpage(event);
> > + }
> > +}
> > +
> > struct riscv_pmu *riscv_pmu_alloc(void)
> > {
> > struct riscv_pmu *pmu;
> > @@ -307,6 +345,9 @@ struct riscv_pmu *riscv_pmu_alloc(void)
> > }
> > pmu->pmu = (struct pmu) {
> > .event_init = riscv_pmu_event_init,
> > + .event_mapped = riscv_pmu_event_mapped,
> > + .event_unmapped = riscv_pmu_event_unmapped,
> > + .event_idx = riscv_pmu_event_idx,
> > .add = riscv_pmu_add,
> > .del = riscv_pmu_del,
> > .start = riscv_pmu_start,
> > diff --git a/include/linux/perf/riscv_pmu.h b/include/linux/perf/riscv_pmu.h
> > index 9f70d94942e0..1452c8af3b67 100644
> > --- a/include/linux/perf/riscv_pmu.h
> > +++ b/include/linux/perf/riscv_pmu.h
> > @@ -55,6 +55,10 @@ struct riscv_pmu {
> > void (*ctr_start)(struct perf_event *event, u64 init_val);
> > void (*ctr_stop)(struct perf_event *event, unsigned long flag);
> > int (*event_map)(struct perf_event *event, u64 *config);
> > + void (*event_init)(struct perf_event *event);
> > + void (*event_mapped)(struct perf_event *event, struct mm_struct *mm);
> > + void (*event_unmapped)(struct perf_event *event, struct mm_struct *mm);
> > + uint8_t (*csr_index)(struct perf_event *event);
> >
> > struct cpu_hw_events __percpu *hw_events;
> > struct hlist_node node;
> > --
> > 2.37.2
> >
>
>
> --
> Regards,
> Atish