Re: [PATCH v3 09/16] perf/core: Use static_call to optimize perf_guest_info_callbacks

Paolo Bonzini
2021-09-22T06:33:22Z

On 22/09/21 02:05, Sean Christopherson wrote:
> Use static_call to optimize perf's guest callbacks on arm64 and x86,
> which are now the only architectures that define the callbacks. Use
> DEFINE_STATIC_CALL_RET0 as the default/NULL for all guest callbacks, as
> the callback semantics are that a return value '0' means "not in guest".
>
> static_call obviously avoids the overhead of CONFIG_RETPOLINE=y, but is
> also advantageous versus other solutions, e.g. per-cpu callbacks, in that
> a per-cpu memory load is not needed to detect the !guest case.
>
> Based on code from Peter and Like.
>
> Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Cc: Like Xu <like.xu.linux@gmail.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> include/linux/perf_event.h | 28 ++++++----------------------
> kernel/events/core.c | 15 +++++++++++++++
> 2 files changed, 21 insertions(+), 22 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index eefa197d5354..d582dfeb4e20 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1240,37 +1240,21 @@ extern void perf_event_bpf_event(struct bpf_prog *prog,
>
> #ifdef CONFIG_GUEST_PERF_EVENTS
> extern struct perf_guest_info_callbacks *perf_guest_cbs;
> -static inline struct perf_guest_info_callbacks *perf_get_guest_cbs(void)
> -{
> - /* Reg/unreg perf_guest_cbs waits for readers via synchronize_rcu(). */
> - lockdep_assert_preemption_disabled();
> +DECLARE_STATIC_CALL(__perf_guest_state, *perf_guest_cbs->state);
> +DECLARE_STATIC_CALL(__perf_guest_get_ip, *perf_guest_cbs->get_ip);
> +DECLARE_STATIC_CALL(__perf_guest_handle_intel_pt_intr, *perf_guest_cbs->handle_intel_pt_intr);
>
> - /* Prevent reloading between a !NULL check and dereferences. */
> - return READ_ONCE(perf_guest_cbs);
> -}
> static inline unsigned int perf_guest_state(void)
> {
> - struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
> -
> - return guest_cbs ? guest_cbs->state() : 0;
> + return static_call(__perf_guest_state)();
> }
> static inline unsigned long perf_guest_get_ip(void)
> {
> - struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
> -
> - /*
> - * Arbitrarily return '0' in the unlikely scenario that the callbacks
> - * are unregistered between checking guest state and getting the IP.
> - */
> - return guest_cbs ? guest_cbs->get_ip() : 0;
> + return static_call(__perf_guest_get_ip)();
> }
> static inline unsigned int perf_guest_handle_intel_pt_intr(void)
> {
> - struct perf_guest_info_callbacks *guest_cbs = perf_get_guest_cbs();
> -
> - if (guest_cbs && guest_cbs->handle_intel_pt_intr)
> - return guest_cbs->handle_intel_pt_intr();
> - return 0;
> + return static_call(__perf_guest_handle_intel_pt_intr)();
> }
> extern void perf_register_guest_info_callbacks(struct perf_guest_info_callbacks *cbs);
> extern void perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs);
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index c6ec05809f54..79c8ee1778a4 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6485,12 +6485,23 @@ static void perf_pending_event(struct irq_work *entry)
> #ifdef CONFIG_GUEST_PERF_EVENTS
> struct perf_guest_info_callbacks *perf_guest_cbs;
>
> +DEFINE_STATIC_CALL_RET0(__perf_guest_state, *perf_guest_cbs->state);
> +DEFINE_STATIC_CALL_RET0(__perf_guest_get_ip, *perf_guest_cbs->get_ip);
> +DEFINE_STATIC_CALL_RET0(__perf_guest_handle_intel_pt_intr, *perf_guest_cbs->handle_intel_pt_intr);
> +
> void perf_register_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
> {
> if (WARN_ON_ONCE(perf_guest_cbs))
> return;
>
> WRITE_ONCE(perf_guest_cbs, cbs);
> + static_call_update(__perf_guest_state, cbs->state);
> + static_call_update(__perf_guest_get_ip, cbs->get_ip);
> +
> + /* Implementing ->handle_intel_pt_intr is optional. */
> + if (cbs->handle_intel_pt_intr)
> + static_call_update(__perf_guest_handle_intel_pt_intr,
> + cbs->handle_intel_pt_intr);
> }
> EXPORT_SYMBOL_GPL(perf_register_guest_info_callbacks);
>
> @@ -6500,6 +6511,10 @@ void perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
> return;
>
> WRITE_ONCE(perf_guest_cbs, NULL);
> + static_call_update(__perf_guest_state, (void *)&__static_call_return0);
> + static_call_update(__perf_guest_get_ip, (void *)&__static_call_return0);
> + static_call_update(__perf_guest_handle_intel_pt_intr,
> + (void *)&__static_call_return0);
> synchronize_rcu();
> }
> EXPORT_SYMBOL_GPL(perf_unregister_guest_info_callbacks);
>

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>