Branch hint profiling on my nehalem machine showed 88%
incorrect branch hints:
42017484 326957902 88 context_switch sched.c 3043
42038493 326953687 88 context_switch sched.c 3050
Signed-off-by: Tim Blechmann <[email protected]>
---
kernel/sched.c | 4 ++--
1 files changed, 2 insertions(+), 2 deletions(-)
On 11/29/2009 02:01 PM, Tim Blechmann wrote:
> Branch hint profiling on my nehalem machine showed 88%
> incorrect branch hints:
>
> 42017484 326957902 88 context_switch sched.c 3043
> 42038493 326953687 88 context_switch sched.c 3050
>
> @@ -3040,14 +3040,14 @@ context_switch(struct rq *rq, struct task_struct *prev,
> */
> arch_start_context_switch(prev);
>
> - if (likely(!mm)) {
> + if (unlikely(!mm)) {
> next->active_mm = oldmm;
> atomic_inc(&oldmm->mm_count);
> enter_lazy_tlb(oldmm, next);
> } else
> switch_mm(oldmm, mm, next);
>
> - if (likely(!prev->mm)) {
> + if (unlikely(!prev->mm)) {
> prev->active_mm = NULL;
> rq->prev_mm = oldmm;
> }
>
I don't think either the original or the patch is correct. Whether or
not a task has an mm is entirely workload dependent, we shouldn't be
giving hints here.
--
error compiling committee.c: too many arguments to function
On Sun, 2009-11-29 at 17:12 +0200, Avi Kivity wrote:
> On 11/29/2009 02:01 PM, Tim Blechmann wrote:
> > Branch hint profiling on my nehalem machine showed 88%
> > incorrect branch hints:
> >
> > 42017484 326957902 88 context_switch sched.c 3043
> > 42038493 326953687 88 context_switch sched.c 3050
> >
> > @@ -3040,14 +3040,14 @@ context_switch(struct rq *rq, struct task_struct *prev,
> > */
> > arch_start_context_switch(prev);
> >
> > - if (likely(!mm)) {
> > + if (unlikely(!mm)) {
> > next->active_mm = oldmm;
> > atomic_inc(&oldmm->mm_count);
> > enter_lazy_tlb(oldmm, next);
> > } else
> > switch_mm(oldmm, mm, next);
> >
> > - if (likely(!prev->mm)) {
> > + if (unlikely(!prev->mm)) {
> > prev->active_mm = NULL;
> > rq->prev_mm = oldmm;
> > }
> >
>
> I don't think either the original or the patch is correct. Whether or
> not a task has an mm is entirely workload dependent, we shouldn't be
> giving hints here.
There are reasons to still use branch hints, for example if the unlikely
branch is very expensive anyway and it pays to have the likely branch be
ever so slightly less expensive.
Now I don't think that applies here, but there are cases where such code
generation issues are the main motivator not the actual usage patterns.
On 11/29/2009 05:20 PM, Peter Zijlstra wrote:
> On Sun, 2009-11-29 at 17:12 +0200, Avi Kivity wrote:
>
>> On 11/29/2009 02:01 PM, Tim Blechmann wrote:
>>
>>> Branch hint profiling on my nehalem machine showed 88%
>>> incorrect branch hints:
>>>
>>> 42017484 326957902 88 context_switch sched.c 3043
>>> 42038493 326953687 88 context_switch sched.c 3050
>>>
>>> @@ -3040,14 +3040,14 @@ context_switch(struct rq *rq, struct task_struct *prev,
>>> */
>>> arch_start_context_switch(prev);
>>>
>>> - if (likely(!mm)) {
>>> + if (unlikely(!mm)) {
>>> next->active_mm = oldmm;
>>> atomic_inc(&oldmm->mm_count);
>>> enter_lazy_tlb(oldmm, next);
>>> } else
>>> switch_mm(oldmm, mm, next);
>>>
>>> - if (likely(!prev->mm)) {
>>> + if (unlikely(!prev->mm)) {
>>> prev->active_mm = NULL;
>>> rq->prev_mm = oldmm;
>>> }
>>>
>>>
>> I don't think either the original or the patch is correct. Whether or
>> not a task has an mm is entirely workload dependent, we shouldn't be
>> giving hints here.
>>
> There are reasons to still use branch hints, for example if the unlikely
> branch is very expensive anyway and it pays to have the likely branch be
> ever so slightly less expensive.
>
> Now I don't think that applies here, but there are cases where such code
> generation issues are the main motivator not the actual usage patterns.
>
These should be documented then to avoid patches removing them:
#define slowpath(x) unlikely(x)
if (slowpath(condition))
expensive_operation();
--
error compiling committee.c: too many arguments to function
On 11/29/2009 04:25 PM, Avi Kivity wrote:
> On 11/29/2009 05:20 PM, Peter Zijlstra wrote:
>> On Sun, 2009-11-29 at 17:12 +0200, Avi Kivity wrote:
>>
>>> On 11/29/2009 02:01 PM, Tim Blechmann wrote:
>>>
>>>> Branch hint profiling on my nehalem machine showed 88%
>>>> incorrect branch hints:
>>>>
>>>> 42017484 326957902 88 context_switch sched.c 3043
>>>> 42038493 326953687 88 context_switch sched.c 3050
>>>>
>>>> @@ -3040,14 +3040,14 @@ context_switch(struct rq *rq, struct task_struct *prev,
>>>> */
>>>> arch_start_context_switch(prev);
>>>>
>>>> - if (likely(!mm)) {
>>>> + if (unlikely(!mm)) {
>>>> next->active_mm = oldmm;
>>>> atomic_inc(&oldmm->mm_count);
>>>> enter_lazy_tlb(oldmm, next);
>>>> } else
>>>> switch_mm(oldmm, mm, next);
>>>>
>>>> - if (likely(!prev->mm)) {
>>>> + if (unlikely(!prev->mm)) {
>>>> prev->active_mm = NULL;
>>>> rq->prev_mm = oldmm;
>>>> }
>>>>
>>>>
>>> I don't think either the original or the patch is correct. Whether or
>>> not a task has an mm is entirely workload dependent, we shouldn't be
>>> giving hints here.
>>>
>> There are reasons to still use branch hints, for example if the unlikely
>> branch is very expensive anyway and it pays to have the likely branch be
>> ever so slightly less expensive.
>>
>> Now I don't think that applies here, but there are cases where such code
>> generation issues are the main motivator not the actual usage patterns.
would be nice, if you commit a patch, removing this hint
> These should be documented then to avoid patches removing them:
>
> #define slowpath(x) unlikely(x)
>
> if (slowpath(condition))
> expensive_operation();
this would definitely improve the expressive power ...
thnx, tim
--
[email protected]
http://tim.klingt.org
Only very good and very bad programmers use goto in C
Am Sonntag 29 November 2009 16:25:43 schrieb Avi Kivity:
> These should be documented then to avoid patches removing them:
>
> #define slowpath(x) unlikely(x)
>
> if (slowpath(condition))
> expensive_operation();
Neat. If we also modify the likelyhood tracer to __not__ check the
slowpath annotation by default this really looks like a good idea.
Christian