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Subject[RESEND PATCH] bpf: Add support for absolute value BPF timers
Date
Add a new flag BPF_F_TIMER_ABS that can be passed to bpf_timer_start()
to start an absolute value timer instead of the default relative value.
This makes the timer expire at an exact point in time, instead of a time
with latencies and jitter induced by both the BPF and timer subsystems.
This is useful e.g. in certain time sensitive profiling cases, where we
need a timer to expire at an exact point in time.

Signed-off-by: Tero Kristo <tero.kristo@linux.intel.com>
---
include/uapi/linux/bpf.h | 15 +++++++++++++++
kernel/bpf/helpers.c | 11 +++++++++--
2 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 464ca3f01fe7..7f5b71847984 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -4951,6 +4951,12 @@ union bpf_attr {
* different maps if key/value layout matches across maps.
* Every bpf_timer_set_callback() can have different callback_fn.
*
+ * *flags* can be one of:
+ *
+ * **BPF_F_TIMER_ABS**
+ * Start the timer in absolute expire value instead of the
+ * default relative one.
+ *
* Return
* 0 on success.
* **-EINVAL** if *timer* was not initialized with bpf_timer_init() earlier
@@ -7050,4 +7056,13 @@ struct bpf_core_relo {
enum bpf_core_relo_kind kind;
};

+/*
+ * Flags to control bpf_timer_start() behaviour.
+ * - BPF_F_TIMER_ABS: Timeout passed is absolute time, by default it is
+ * relative to current time.
+ */
+enum {
+ BPF_F_TIMER_ABS = (1ULL << 0),
+};
+
#endif /* _UAPI__LINUX_BPF_H__ */
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index af30c6cbd65d..924849d89828 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1253,10 +1253,11 @@ BPF_CALL_3(bpf_timer_start, struct bpf_timer_kern *, timer, u64, nsecs, u64, fla
{
struct bpf_hrtimer *t;
int ret = 0;
+ enum hrtimer_mode mode;

if (in_nmi())
return -EOPNOTSUPP;
- if (flags)
+ if (flags > BPF_F_TIMER_ABS)
return -EINVAL;
__bpf_spin_lock_irqsave(&timer->lock);
t = timer->timer;
@@ -1264,7 +1265,13 @@ BPF_CALL_3(bpf_timer_start, struct bpf_timer_kern *, timer, u64, nsecs, u64, fla
ret = -EINVAL;
goto out;
}
- hrtimer_start(&t->timer, ns_to_ktime(nsecs), HRTIMER_MODE_REL_SOFT);
+
+ if (flags & BPF_F_TIMER_ABS)
+ mode = HRTIMER_MODE_ABS_SOFT;
+ else
+ mode = HRTIMER_MODE_REL_SOFT;
+
+ hrtimer_start(&t->timer, ns_to_ktime(nsecs), mode);
out:
__bpf_spin_unlock_irqrestore(&timer->lock);
return ret;
--
2.25.1
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