| Message ID | 1431549503-11799-1-git-send-email-jorge.ramirez-ortiz@linaro.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Hi Jorge, On 13 May 2015 at 22:38, Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org> wrote: > This commit sets the capacity of the average CPU in SMP systems to > SCHED_CAPACITY_SCALE. > > Ignoring the condition "min_capacity==max_capacity" causes the function > update_cpu_capacity( .. ) to generate out of range values [1]. This is > because the default value of middle_capacity is used in the final > calculation instead of a valid scaling factor. > > Incidentally, when out of range values are generated and if > SCHED_FEAT(ARCH_POWER, true), the load balancing algorithm makes the > wrong decisions typically overallocating work on one of the cores > while leaving the others unused. Have you got an example ? > > [1] val > SCHED_CAPACITY_SCALE > > Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org> > --- > arch/arm/kernel/topology.c | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/arch/arm/kernel/topology.c b/arch/arm/kernel/topology.c > index 08b7847..509bc9b 100644 > --- a/arch/arm/kernel/topology.c > +++ b/arch/arm/kernel/topology.c > @@ -137,14 +137,14 @@ static void __init parse_dt_topology(void) > cpu_capacity(cpu) = capacity; > } > > - /* If min and max capacities are equals, we bypass the update of the > - * cpu_scale because all CPUs have the same capacity. Otherwise, we > - * compute a middle_capacity factor that will ensure that the capacity > + /* Compute a middle_capacity factor that will ensure that the capacity > * of an 'average' CPU of the system will be as close as possible to > * SCHED_CAPACITY_SCALE, which is the default value, but with the > * constraint explained near table_efficiency[]. > */ > - if (4*max_capacity < (3*(max_capacity + min_capacity))) > + if (min_capacity == max_capacity) > + middle_capacity = min_capacity >> SCHED_CAPACITY_SHIFT; > + else if (4*max_capacity < (3*(max_capacity + min_capacity))) if min_capacity == max_capacity then the condition 4*max_capacity < (3*(max_capacity + min_capacity)) is true and middle_capacity = (min_capacity + max_capacity) >> (SCHED_CAPACITY_SHIFT+1) = 2*min_capacity >> (SCHED_CAPACITY_SHIFT+1) so middle capacity = min_capacity >> SCHED_CAPACITY_SHIFT I don't see what your change does that is not already done by current code Regards, Vincent > middle_capacity = (min_capacity + max_capacity) > >> (SCHED_CAPACITY_SHIFT+1); > else > -- > 2.1.4 >
On 05/14/2015 07:31 AM, Vincent Guittot wrote: > Hi Jorge, > > On 13 May 2015 at 22:38, Jorge Ramirez-Ortiz > <jorge.ramirez-ortiz@linaro.org> wrote: >> This commit sets the capacity of the average CPU in SMP systems to >> SCHED_CAPACITY_SCALE. >> >> Ignoring the condition "min_capacity==max_capacity" causes the function >> update_cpu_capacity( .. ) to generate out of range values [1]. This is >> because the default value of middle_capacity is used in the final >> calculation instead of a valid scaling factor. >> >> Incidentally, when out of range values are generated and if >> SCHED_FEAT(ARCH_POWER, true), the load balancing algorithm makes the >> wrong decisions typically overallocating work on one of the cores >> while leaving the others unused. > Have you got an example ? This was tested on lsk 3.10 which was slightly different (see my comments below). I can get you the test source code. > >> [1] val > SCHED_CAPACITY_SCALE >> >> Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org> >> --- >> arch/arm/kernel/topology.c | 8 ++++---- >> 1 file changed, 4 insertions(+), 4 deletions(-) >> >> diff --git a/arch/arm/kernel/topology.c b/arch/arm/kernel/topology.c >> index 08b7847..509bc9b 100644 >> --- a/arch/arm/kernel/topology.c >> +++ b/arch/arm/kernel/topology.c >> @@ -137,14 +137,14 @@ static void __init parse_dt_topology(void) >> cpu_capacity(cpu) = capacity; >> } >> >> - /* If min and max capacities are equals, we bypass the update of the >> - * cpu_scale because all CPUs have the same capacity. Otherwise, we >> - * compute a middle_capacity factor that will ensure that the capacity >> + /* Compute a middle_capacity factor that will ensure that the capacity >> * of an 'average' CPU of the system will be as close as possible to >> * SCHED_CAPACITY_SCALE, which is the default value, but with the >> * constraint explained near table_efficiency[]. >> */ >> - if (4*max_capacity < (3*(max_capacity + min_capacity))) >> + if (min_capacity == max_capacity) >> + middle_capacity = min_capacity >> SCHED_CAPACITY_SHIFT; >> + else if (4*max_capacity < (3*(max_capacity + min_capacity))) > if min_capacity == max_capacity then the condition 4*max_capacity < > (3*(max_capacity + min_capacity)) is true and > middle_capacity = (min_capacity + max_capacity) >> > (SCHED_CAPACITY_SHIFT+1) = 2*min_capacity >> (SCHED_CAPACITY_SHIFT+1) > so middle capacity = min_capacity >> SCHED_CAPACITY_SHIFT > > I don't see what your change does that is not already done by current code ah you are right! I didn't even consider it would be handled under that condition. My mistake. It seem the problem is only present in lsk 3.10 then [1] btw the comments above the condition should still be edited (cpu_scale is always updated) ok ignore this patch and let's only fix it on the lsk in the same way it is done here. thanks [1] https://git.linaro.org/kernel/linux-linaro-stable.git/blob/refs/heads/linux-linaro-lsk:/arch/arm64/kernel/topology.c#l327
diff --git a/arch/arm/kernel/topology.c b/arch/arm/kernel/topology.c index 08b7847..509bc9b 100644 --- a/arch/arm/kernel/topology.c +++ b/arch/arm/kernel/topology.c @@ -137,14 +137,14 @@ static void __init parse_dt_topology(void) cpu_capacity(cpu) = capacity; } - /* If min and max capacities are equals, we bypass the update of the - * cpu_scale because all CPUs have the same capacity. Otherwise, we - * compute a middle_capacity factor that will ensure that the capacity + /* Compute a middle_capacity factor that will ensure that the capacity * of an 'average' CPU of the system will be as close as possible to * SCHED_CAPACITY_SCALE, which is the default value, but with the * constraint explained near table_efficiency[]. */ - if (4*max_capacity < (3*(max_capacity + min_capacity))) + if (min_capacity == max_capacity) + middle_capacity = min_capacity >> SCHED_CAPACITY_SHIFT; + else if (4*max_capacity < (3*(max_capacity + min_capacity))) middle_capacity = (min_capacity + max_capacity) >> (SCHED_CAPACITY_SHIFT+1); else
This commit sets the capacity of the average CPU in SMP systems to SCHED_CAPACITY_SCALE. Ignoring the condition "min_capacity==max_capacity" causes the function update_cpu_capacity( .. ) to generate out of range values [1]. This is because the default value of middle_capacity is used in the final calculation instead of a valid scaling factor. Incidentally, when out of range values are generated and if SCHED_FEAT(ARCH_POWER, true), the load balancing algorithm makes the wrong decisions typically overallocating work on one of the cores while leaving the others unused. [1] val > SCHED_CAPACITY_SCALE Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org> --- arch/arm/kernel/topology.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-)