Hi Anna-Maria,
Nice change, it indeed makes more sense that way.
Just a few details below:
On Fri, Jul 10, 2020 at 05:46:22PM +0200, Anna-Maria Behnsen wrote:
> The bucket expiry time is the effective expriy time of timers and is
> greater than or equal to the requested timer expiry time. This is due
> to the guarantee that timers never expire early and the reduced expiry
> granularity in the secondary wheel levels.
>
> When a timer is enqueued, trigger_dyntick_cpu() checks whether the
> timer is the new first timer. This check compares next_expiry with
> the requested timer expiry value and not with the effective expiry
> value of the bucket into which the timer was queued.
>
> Storing the requested timer expiry value in base->next_expiry can lead
> to base->clk going backwards if the requested timer expiry value is
> smaller than base->clk. Commit 30c66fc30ee7 ("timer: Prevent base->clk
> from moving backward") worked around this by preventing the store when
> timer->expiry is before base->clk, but did not fix the underlying
> problem.
>
> Use the expiry value of the bucket into which the timer is queued to
> do the new first timer check. This fixes the base->clk going backward
> problem and also prevents unnecessary softirq invocations when the
> timer->expiry is not equal to the bucket expiry time in case of a new
> first timer which is queued in a secondary wheel level.
I think there shouldn't be such unecessary softirq invocations. Either they
fire at the bucket expiry time or the timer expiry time, it doesn't make
much difference.
More important below:
> -static int calc_wheel_index(unsigned long expires, unsigned long clk)
> +static int calc_wheel_index(unsigned long expires, unsigned long clk,
> + unsigned long *bucket_expiry)
> {
> unsigned long delta = expires - clk;
> unsigned int idx;
>
> if (delta < LVL_START(1)) {
> - idx = calc_index(expires, 0);
> + idx = calc_index(expires, 0, bucket_expiry);
> } else if (delta < LVL_START(2)) {
> - idx = calc_index(expires, 1);
> + idx = calc_index(expires, 1, bucket_expiry);
> } else if (delta < LVL_START(3)) {
> - idx = calc_index(expires, 2);
> + idx = calc_index(expires, 2, bucket_expiry);
> } else if (delta < LVL_START(4)) {
> - idx = calc_index(expires, 3);
> + idx = calc_index(expires, 3, bucket_expiry);
> } else if (delta < LVL_START(5)) {
> - idx = calc_index(expires, 4);
> + idx = calc_index(expires, 4, bucket_expiry);
> } else if (delta < LVL_START(6)) {
> - idx = calc_index(expires, 5);
> + idx = calc_index(expires, 5, bucket_expiry);
> } else if (delta < LVL_START(7)) {
> - idx = calc_index(expires, 6);
> + idx = calc_index(expires, 6, bucket_expiry);
> } else if (LVL_DEPTH > 8 && delta < LVL_START(8)) {
> - idx = calc_index(expires, 7);
> + idx = calc_index(expires, 7, bucket_expiry);
> } else if ((long) delta < 0) {
> idx = clk & LVL_MASK;
You also need to handle that part. That's in fact the critical one :)
I'll rebase my series on top of that.
Thanks!
