Messages in this thread Patch in this message | | | Date | Tue, 12 May 2020 13:21:58 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 0/3] arm64: perf_event: Fix time offset prior to epoch |
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On Tue, May 12, 2020 at 11:19:18AM +0200, Peter Zijlstra wrote:
> Now, your arm64 counter is 56 bits, so wrapping is rare, but still, we > should probably fix that. And that probably needs an ABI extention > *sigh*.
Something like so, on top of the other (2).
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--- a/arch/arm64/kernel/perf_event.c +++ b/arch/arm64/kernel/perf_event.c @@ -1174,6 +1174,7 @@ void arch_perf_update_userpage(struct pe userpg->cap_user_time = 0; userpg->cap_user_time_zero = 0; + userpg->cap_user_time_short = 0; do { rd = sched_clock_read_begin(&seq); @@ -1184,13 +1185,16 @@ void arch_perf_update_userpage(struct pe userpg->time_mult = rd->mult; userpg->time_shift = rd->shift; userpg->time_zero = rd->epoch_ns; + userpg->time_cycle = rd->epoch_cyc; + userpg->time_mask = rd->sched_clock_mask; /* * This isn't strictly correct, the ARM64 counter can be - * 'short' and then we get funnies when it wraps. The correct - * thing would be to extend the perf ABI with a cycle and mask - * value, but because wrapping on ARM64 is very rare in - * practise this 'works'. + * 'short' and then we get funnies when it wraps. But this + * 'works' with the cap_user_time ABI. + * + * When userspace knows about cap_user_time_short, it + * can do the correct thing. */ userpg->time_zero -= (rd->epoch_cyc * rd->mult) >> rd->shift; @@ -1215,4 +1219,5 @@ void arch_perf_update_userpage(struct pe */ userpg->cap_user_time = 1; userpg->cap_user_time_zero = 1; + userpg->cap_user_time_short = 1; } --- a/include/uapi/linux/perf_event.h +++ b/include/uapi/linux/perf_event.h @@ -532,9 +532,10 @@ struct perf_event_mmap_page { cap_bit0_is_deprecated : 1, /* Always 1, signals that bit 0 is zero */ cap_user_rdpmc : 1, /* The RDPMC instruction can be used to read counts */ - cap_user_time : 1, /* The time_* fields are used */ + cap_user_time : 1, /* The time_{shift,mult,offset} fields are used */ cap_user_time_zero : 1, /* The time_zero field is used */ - cap_____res : 59; + cap_user_time_short : 1, /* the time_{cycle,mask} fields are used */ + cap_____res : 58; }; }; @@ -593,13 +594,29 @@ struct perf_event_mmap_page { * ((rem * time_mult) >> time_shift); */ __u64 time_zero; + __u32 size; /* Header size up to __reserved[] fields. */ + __u32 __reserved_1; + + /* + * If cap_usr_time_short, the hardware clock is less than 64bit wide + * and we must compute the 'cyc' value, as used by cap_usr_time, as: + * + * cyc = time_cycles + ((cyc - time_cycles) & time_mask) + * + * NOTE: this form is explicitly chosen such that cap_usr_time_short + * is an extention on top of cap_usr_time, and code that doesn't + * know about cap_usr_time_short still works under the assumption + * the counter doesn't wrap. + */ + __u64 time_cycles; + __u64 time_mask; /* * Hole for extension of the self monitor capabilities */ - __u8 __reserved[118*8+4]; /* align to 1k. */ + __u8 __reserved[116*8]; /* align to 1k. */ /* * Control data for the mmap() data buffer.
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