[PATCH] um: time-travel: actually apply "free-until" optimisation

[PATCH] um: time-travel: actually apply "free-until" optimisation

[Johannes Berg]

From: Johannes Berg <johannes.berg@intel.com>

Due a bug - we never checked the time_travel_ext_free_until value - we
were always requesting time for every single scheduling. This adds up
since we make reading time cost 256ns, and it's a fairly common call.
Fix this.

While at it, also make reading time only cost something when we're not
currently waiting for our scheduling turn - otherwise things get mixed
up in a very confusing way. We should never get here, since we're not
actually running, but it's possible if you stick printk() or such into
the virtio code that must handle the external interrupts.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
---
 arch/um/kernel/time.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/um/kernel/time.c b/arch/um/kernel/time.c
index 303565ce8c64..5ac084deac7e 100644
--- a/arch/um/kernel/time.c
+++ b/arch/um/kernel/time.c
@@ -183,6 +183,14 @@ static void time_travel_ext_update_request(unsigned long long time)
 	    time == time_travel_ext_prev_request)
 		return;
 
+	/*
+	 * if we're running and are allowed to run past the request
+	 * then we don't need to update it either
+	 */
+	if (!time_travel_ext_waiting && time_travel_ext_free_until_valid &&
+	    time < time_travel_ext_free_until)
+		return;
+
 	time_travel_ext_prev_request = time;
 	time_travel_ext_prev_request_valid = true;
 	time_travel_ext_req(UM_TIMETRAVEL_REQUEST, time);
@@ -217,6 +225,7 @@ static void time_travel_ext_wait(bool idle)
 	};
 
 	time_travel_ext_prev_request_valid = false;
+	time_travel_ext_free_until_valid = false;
 	time_travel_ext_waiting++;
 
 	time_travel_ext_req(UM_TIMETRAVEL_WAIT, -1);
@@ -639,7 +648,8 @@ static u64 timer_read(struct clocksource *cs)
 		 * "what do I do next" and onstack event we use to know when
 		 * to return from time_travel_update_time().
 		 */
-		if (!irqs_disabled() && !in_interrupt() && !in_softirq())
+		if (!irqs_disabled() && !in_interrupt() && !in_softirq() &&
+		    !time_travel_ext_waiting)
 			time_travel_update_time(time_travel_time +
 						TIMER_MULTIPLIER,
 						false);
-- 
2.26.2


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Re: [PATCH] um: time-travel: actually apply "free-until" optimisation

[Richard Weinberger]

On Thu, Dec 3, 2020 at 3:37 PM Johannes Berg <johannes@sipsolutions.net> wrote:
>
> From: Johannes Berg <johannes.berg@intel.com>
>
> Due a bug - we never checked the time_travel_ext_free_until value - we
> were always requesting time for every single scheduling. This adds up
> since we make reading time cost 256ns, and it's a fairly common call.
> Fix this.
>
> While at it, also make reading time only cost something when we're not
> currently waiting for our scheduling turn - otherwise things get mixed
> up in a very confusing way. We should never get here, since we're not
> actually running, but it's possible if you stick printk() or such into
> the virtio code that must handle the external interrupts.
>
> Signed-off-by: Johannes Berg <johannes.berg@intel.com>
> ---
>  arch/um/kernel/time.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/arch/um/kernel/time.c b/arch/um/kernel/time.c
> index 303565ce8c64..5ac084deac7e 100644
> --- a/arch/um/kernel/time.c
> +++ b/arch/um/kernel/time.c
> @@ -183,6 +183,14 @@ static void time_travel_ext_update_request(unsigned long long time)
>             time == time_travel_ext_prev_request)
>                 return;
>
> +       /*
> +        * if we're running and are allowed to run past the request
> +        * then we don't need to update it either
> +        */
> +       if (!time_travel_ext_waiting && time_travel_ext_free_until_valid &&
> +           time < time_travel_ext_free_until)
> +               return;
> +
>         time_travel_ext_prev_request = time;
>         time_travel_ext_prev_request_valid = true;
>         time_travel_ext_req(UM_TIMETRAVEL_REQUEST, time);
> @@ -217,6 +225,7 @@ static void time_travel_ext_wait(bool idle)
>         };
>
>         time_travel_ext_prev_request_valid = false;
> +       time_travel_ext_free_until_valid = false;
>         time_travel_ext_waiting++;
>
>         time_travel_ext_req(UM_TIMETRAVEL_WAIT, -1);
> @@ -639,7 +648,8 @@ static u64 timer_read(struct clocksource *cs)
>                  * "what do I do next" and onstack event we use to know when
>                  * to return from time_travel_update_time().
>                  */
> -               if (!irqs_disabled() && !in_interrupt() && !in_softirq())
> +               if (!irqs_disabled() && !in_interrupt() && !in_softirq() &&
> +                   !time_travel_ext_waiting)

This will not build with timetravel support disabled.

>                         time_travel_update_time(time_travel_time +
>                                                 TIMER_MULTIPLIER,
>                                                 false);
> --
> 2.26.2
>
>
> _______________________________________________
> linux-um mailing list
> linux-um@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-um



-- 
Thanks,
//richard

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Re: [PATCH] um: time-travel: actually apply "free-until" optimisation

[Johannes Berg]

On Thu, 2020-12-10 at 23:28 +0100, Richard Weinberger wrote:
> 
> > -               if (!irqs_disabled() && !in_interrupt() && !in_softirq())
> > +               if (!irqs_disabled() && !in_interrupt() && !in_softirq() &&
> > +                   !time_travel_ext_waiting)
> 
> This will not build with timetravel support disabled.

Ahrg, sorry, will fix.

johannes


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