bpf: Add ability to pin bpf timer to calling CPU

BPF supports creating high resolution timers using bpf_timer_* helper
functions. Currently, only the BPF_F_TIMER_ABS flag is supported, which
specifies that the timeout should be interpreted as absolute time. It
would also be useful to be able to pin that timer to a core. For
example, if you wanted to make a subset of cores run without timer
interrupts, and only have the timer be invoked on a single core.

This patch adds support for this with a new BPF_F_TIMER_CPU_PIN flag.
When specified, the HRTIMER_MODE_PINNED flag is passed to
hrtimer_start(). A subsequent patch will update selftests to validate.

Signed-off-by: David Vernet <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Song Liu <[email protected]>
Acked-by: Hou Tao <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

include/uapi/linux/bpf.h

@@ -5096,6 +5096,8 @@ union bpf_attr {
  *		**BPF_F_TIMER_ABS**
  *			Start the timer in absolute expire value instead of the
  *			default relative one.
+ *		**BPF_F_TIMER_CPU_PIN**
+ *			Timer will be pinned to the CPU of the caller.
  *
  *	Return
  *		0 on success.
@@ -7309,9 +7311,11 @@ struct bpf_core_relo {
  * Flags to control bpf_timer_start() behaviour.
  *     - BPF_F_TIMER_ABS: Timeout passed is absolute time, by default it is
  *       relative to current time.
+ *     - BPF_F_TIMER_CPU_PIN: Timer will be pinned to the CPU of the caller.
  */
 enum {
 	BPF_F_TIMER_ABS = (1ULL << 0),
+	BPF_F_TIMER_CPU_PIN = (1ULL << 1),
 };
 
 /* BPF numbers iterator state */