Re: [PATCH] sched/rt: Do not do push/pull when there is only one CPU
On Sat, 2 Dec 2017 12:12:52 -0500 Steven Rostedtwrote: > v2 coming up. Not really a v2, but a different approach. Look for the patch with the subject: "[PATCH] sched/rt: Do not pull from current CPU if only one cpu to pull" -- Steve
Re: [PATCH] sched/rt: Do not do push/pull when there is only one CPU
On Sat, 2 Dec 2017 12:12:52 -0500 Steven Rostedt wrote: > v2 coming up. Not really a v2, but a different approach. Look for the patch with the subject: "[PATCH] sched/rt: Do not pull from current CPU if only one cpu to pull" -- Steve
Re: [PATCH] sched/rt: Do not do push/pull when there is only one CPU
On Sat, 2 Dec 2017 13:53:31 +0100 Sebastian Andrzej Siewiorwrote: > what about a check next to sched_feat(RT_PUSH_IPI)? I don't know if this > is a hot path or not (due to bitmap_weight). If it is, then I would > suggest something like a jump-label which is enabled if more than one > CPU has been enabled on boot. Yeah I didn't like that because of the overhead. But I was being optimistic that the cpu weight function would be a nit to the actual pull logic. But I have a better plan. I would like to disable RT_PUSH_IPI, but that's a sched feature and that is a constant if we build without sched debugging. v2 coming up. -- Steve
Re: [PATCH] sched/rt: Do not do push/pull when there is only one CPU
On Sat, 2 Dec 2017 13:53:31 +0100 Sebastian Andrzej Siewior wrote: > what about a check next to sched_feat(RT_PUSH_IPI)? I don't know if this > is a hot path or not (due to bitmap_weight). If it is, then I would > suggest something like a jump-label which is enabled if more than one > CPU has been enabled on boot. Yeah I didn't like that because of the overhead. But I was being optimistic that the cpu weight function would be a nit to the actual pull logic. But I have a better plan. I would like to disable RT_PUSH_IPI, but that's a sched feature and that is a constant if we build without sched debugging. v2 coming up. -- Steve
Re: [PATCH] sched/rt: Do not do push/pull when there is only one CPU
On Sat, Dec 02, 2017 at 01:53:31PM +0100, Sebastian Andrzej Siewior wrote: > On 2017-12-01 13:32:22 [-0500], Steven Rostedt wrote: > > > > Daniel Wagner reported a crash on the beaglebone black. This is a > > single CPU architecture, and does not have a functional: > > arch_send_call_function_single_ipi() and can crash if that is called. > > > > As it only has one CPU, it shouldn't be called, but if the kernel is > > compiled for SMP, the push/pull RT scheduling logic now calls it for > > irq_work if the one CPU is overloaded, it can use that function to call > > itself and crash the kernel. > > > > There's no reason for the push/pull logic to even be called if there's > > only one CPU online. Have it bail if it sees that's the case. > > > > Link: > > http://lkml.kernel.org/r/8c913cc2-b2e3-8c2e-e503-aff1428f8...@monom.org > > Fixes: 4bdced5c9 ("sched/rt: Simplify the IPI based RT balancing logic") > > Cc: sta...@vger.kernel.org > > Reported-by: Daniel Wagner> > --- > > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > > index 4056c19ca3f0..50d2f8179f70 100644 > > --- a/kernel/sched/rt.c > > +++ b/kernel/sched/rt.c > > @@ -1784,6 +1784,10 @@ static int push_rt_task(struct rq *rq) > > if (!rq->rt.overloaded) > > return 0; > > > > + /* If we are the only CPU, don't bother */ > > + if (num_online_cpus() == 1) > > + return 0; > > + > > what about a check next to sched_feat(RT_PUSH_IPI)? I don't know if this > is a hot path or not (due to bitmap_weight). If it is, then I would > suggest something like a jump-label which is enabled if more than one > CPU has been enabled on boot. Yeah good point; bitmap_weight can be quite expensive.
Re: [PATCH] sched/rt: Do not do push/pull when there is only one CPU
On Sat, Dec 02, 2017 at 01:53:31PM +0100, Sebastian Andrzej Siewior wrote: > On 2017-12-01 13:32:22 [-0500], Steven Rostedt wrote: > > > > Daniel Wagner reported a crash on the beaglebone black. This is a > > single CPU architecture, and does not have a functional: > > arch_send_call_function_single_ipi() and can crash if that is called. > > > > As it only has one CPU, it shouldn't be called, but if the kernel is > > compiled for SMP, the push/pull RT scheduling logic now calls it for > > irq_work if the one CPU is overloaded, it can use that function to call > > itself and crash the kernel. > > > > There's no reason for the push/pull logic to even be called if there's > > only one CPU online. Have it bail if it sees that's the case. > > > > Link: > > http://lkml.kernel.org/r/8c913cc2-b2e3-8c2e-e503-aff1428f8...@monom.org > > Fixes: 4bdced5c9 ("sched/rt: Simplify the IPI based RT balancing logic") > > Cc: sta...@vger.kernel.org > > Reported-by: Daniel Wagner > > --- > > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > > index 4056c19ca3f0..50d2f8179f70 100644 > > --- a/kernel/sched/rt.c > > +++ b/kernel/sched/rt.c > > @@ -1784,6 +1784,10 @@ static int push_rt_task(struct rq *rq) > > if (!rq->rt.overloaded) > > return 0; > > > > + /* If we are the only CPU, don't bother */ > > + if (num_online_cpus() == 1) > > + return 0; > > + > > what about a check next to sched_feat(RT_PUSH_IPI)? I don't know if this > is a hot path or not (due to bitmap_weight). If it is, then I would > suggest something like a jump-label which is enabled if more than one > CPU has been enabled on boot. Yeah good point; bitmap_weight can be quite expensive.
Re: [PATCH] sched/rt: Do not do push/pull when there is only one CPU
On 2017-12-01 13:32:22 [-0500], Steven Rostedt wrote: > > Daniel Wagner reported a crash on the beaglebone black. This is a > single CPU architecture, and does not have a functional: > arch_send_call_function_single_ipi() and can crash if that is called. > > As it only has one CPU, it shouldn't be called, but if the kernel is > compiled for SMP, the push/pull RT scheduling logic now calls it for > irq_work if the one CPU is overloaded, it can use that function to call > itself and crash the kernel. > > There's no reason for the push/pull logic to even be called if there's > only one CPU online. Have it bail if it sees that's the case. > > Link: http://lkml.kernel.org/r/8c913cc2-b2e3-8c2e-e503-aff1428f8...@monom.org > Fixes: 4bdced5c9 ("sched/rt: Simplify the IPI based RT balancing logic") > Cc: sta...@vger.kernel.org > Reported-by: Daniel Wagner> --- > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > index 4056c19ca3f0..50d2f8179f70 100644 > --- a/kernel/sched/rt.c > +++ b/kernel/sched/rt.c > @@ -1784,6 +1784,10 @@ static int push_rt_task(struct rq *rq) > if (!rq->rt.overloaded) > return 0; > > + /* If we are the only CPU, don't bother */ > + if (num_online_cpus() == 1) > + return 0; > + what about a check next to sched_feat(RT_PUSH_IPI)? I don't know if this is a hot path or not (due to bitmap_weight). If it is, then I would suggest something like a jump-label which is enabled if more than one CPU has been enabled on boot. > next_task = pick_next_pushable_task(rq); > if (!next_task) > return 0; > @@ -2038,6 +2042,10 @@ static void pull_rt_task(struct rq *this_rq) > if (likely(!rt_overloaded(this_rq))) > return; > > + /* If we are the only CPU, don't bother */ > + if (num_online_cpus() == 1) > + return; > + > /* >* Match the barrier from rt_set_overloaded; this guarantees that if we >* see overloaded we must also see the rto_mask bit. Sebastian
Re: [PATCH] sched/rt: Do not do push/pull when there is only one CPU
On 2017-12-01 13:32:22 [-0500], Steven Rostedt wrote: > > Daniel Wagner reported a crash on the beaglebone black. This is a > single CPU architecture, and does not have a functional: > arch_send_call_function_single_ipi() and can crash if that is called. > > As it only has one CPU, it shouldn't be called, but if the kernel is > compiled for SMP, the push/pull RT scheduling logic now calls it for > irq_work if the one CPU is overloaded, it can use that function to call > itself and crash the kernel. > > There's no reason for the push/pull logic to even be called if there's > only one CPU online. Have it bail if it sees that's the case. > > Link: http://lkml.kernel.org/r/8c913cc2-b2e3-8c2e-e503-aff1428f8...@monom.org > Fixes: 4bdced5c9 ("sched/rt: Simplify the IPI based RT balancing logic") > Cc: sta...@vger.kernel.org > Reported-by: Daniel Wagner > --- > diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > index 4056c19ca3f0..50d2f8179f70 100644 > --- a/kernel/sched/rt.c > +++ b/kernel/sched/rt.c > @@ -1784,6 +1784,10 @@ static int push_rt_task(struct rq *rq) > if (!rq->rt.overloaded) > return 0; > > + /* If we are the only CPU, don't bother */ > + if (num_online_cpus() == 1) > + return 0; > + what about a check next to sched_feat(RT_PUSH_IPI)? I don't know if this is a hot path or not (due to bitmap_weight). If it is, then I would suggest something like a jump-label which is enabled if more than one CPU has been enabled on boot. > next_task = pick_next_pushable_task(rq); > if (!next_task) > return 0; > @@ -2038,6 +2042,10 @@ static void pull_rt_task(struct rq *this_rq) > if (likely(!rt_overloaded(this_rq))) > return; > > + /* If we are the only CPU, don't bother */ > + if (num_online_cpus() == 1) > + return; > + > /* >* Match the barrier from rt_set_overloaded; this guarantees that if we >* see overloaded we must also see the rto_mask bit. Sebastian