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).
---
--- 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.
