On Thu, Aug 14, 2025 at 06:05:57PM +0200, Igor Mammedov wrote:
> Date: Thu, 14 Aug 2025 18:05:57 +0200
> From: Igor Mammedov <[email protected]>
> Subject: [PATCH v4 5/8] hpet: make main counter read lock-less
>
> Make access to main HPET counter lock-less.
>
> In unlikely event of an update in progress, readers will busy wait
> untill update is finished.
>
> As result micro benchmark of concurrent reading of HPET counter
> with large number of vCPU shows over 80% better (less) latency.
>
> Signed-off-by: Igor Mammedov <[email protected]>
> Reviewed-by: Peter Xu <[email protected]>
> ---
> v3:
> * make reader busy wait during update and reuse existing seqlock API
> Peter Xu <[email protected]>
> ---
> hw/timer/hpet.c | 26 ++++++++++++++++++++------
> 1 file changed, 20 insertions(+), 6 deletions(-)
...
> - QEMU_LOCK_GUARD(&s->lock);
> if (addr == HPET_COUNTER) {
> - if (hpet_enabled(s)) {
> - cur_tick = hpet_get_ticks(s);
> - } else {
> - cur_tick = s->hpet_counter;
> - }
> + unsigned version;
> +
> + /*
> + * Write update is rare, so busywait here is unlikely to happen
> + */
> + do {
> + version = seqlock_read_begin(&s->state_version);
> + if (unlikely(!hpet_enabled(s))) {
is there any particular consideration for rearranging the order of the
conditional branches here (and not directly using likely(hpet_enable()))?
> + cur_tick = s->hpet_counter;
> + } else {
> + cur_tick = hpet_get_ticks(s);
> + }
> + } while (seqlock_read_retry(&s->state_version, version));
> trace_hpet_ram_read_reading_counter(addr & 4, cur_tick);
> return cur_tick >> shift;
> }
Nice imprvoment!
Reviewed-by: Zhao Liu <[email protected]>
