Messages in this thread Patch in this message | | | From | KOSAKI Motohiro <> | Subject | Re: [PATCH] cpumask: convert cpumask_of_cpu() with cpumask_of() | Date | Tue, 26 Apr 2011 20:33:23 +0900 (JST) |
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> On Mon, 2011-04-25 at 18:41 +0900, KOSAKI Motohiro wrote: > > This patch adapt the code to new api fashion. > > > > Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> > > Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> > > Cc: Mike Galbraith <efault@gmx.de> > > Cc: Ingo Molnar <mingo@elte.hu> > > --- > > kernel/kthread.c | 2 +- > > 1 files changed, 1 insertions(+), 1 deletions(-) > > > > diff --git a/kernel/kthread.c b/kernel/kthread.c > > index 3b34d27..4102518 100644 > > --- a/kernel/kthread.c > > +++ b/kernel/kthread.c > > @@ -202,7 +202,7 @@ void kthread_bind(struct task_struct *p, unsigned int cpu) > > return; > > } > > > > - p->cpus_allowed = cpumask_of_cpu(cpu); > > + cpumask_copy(&p->cpus_allowed, cpumask_of(cpu)); > > p->rt.nr_cpus_allowed = 1; > > p->flags |= PF_THREAD_BOUND; > > } > > But why? Are we going to get rid of cpumask_t (which is a fixed sized > struct to direct assignment is perfectly fine)? > > Also, do we want to convert cpus_allowed to cpumask_var_t? It would save > quite a lot of memory on distro configs that set NR_CPUS silly high. > Currently NR_CPUS=4096 configs allocate 512 bytes per task for this > bitmap, 511 of which will never be used on most machines (510 in the > near future). > > The cost if of course an extra memory dereference in scheduler hot > paths.. also not nice.
To be honest, I dislike current cpumask_size() always return NR_CPUS. It screw up almost all of cpumask_var_t benefit. But, we have to eliminate all dangerous = operator usage before changing its implementation.
(btw, I wonder this limitation doesn't documented at all in code. Should we add this?)
Thus, now I'm finding all of =operator by tool and replacing it. The second motivation of eliminating old api is to easy detect obsolete usage by tool.
So, I personally hope task->cpus_allowed convert to cpumask_var_t as cpuset->cpus_allowed. For some years, storage guys repeatedly alert stack overflow chance is increasing as time goes.
However, yes, extra memory dereference is also bad. If scheduler folks dislike cpumask_var_t, I can drop to think convert task->cpus_allowed.
But, if we can't convert cpus_allowed, I'd like to move it into last of task_struct. because usually cpu_allowed is only accessed first byte (as you described).
Thanks.
From 8844bba7597ac1c7fd2e88406da17d818ce271d1 Mon Sep 17 00:00:00 2001 From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Date: Tue, 26 Apr 2011 19:58:39 +0900 Subject: [PATCH] cpumask: add cpumask_var_t documentation
cpumask_var_t has one nortable difference against cpumask_t. This patch adds the explanation.
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> --- include/linux/cpumask.h | 6 ++++++ 1 files changed, 6 insertions(+), 0 deletions(-)
diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h index a3ff24f..3d09505 100644 --- a/include/linux/cpumask.h +++ b/include/linux/cpumask.h @@ -617,6 +617,12 @@ static inline size_t cpumask_size(void) * ... use 'tmpmask' like a normal struct cpumask * ... * * free_cpumask_var(tmpmask); + * + * However, one notable exception is there. cpumask_var_t is allocated + * only nr_cpu_ids bits (in the other hand, real cpumask_t always has + * NR_CPUS bits). therefore cpumask_var_t can't use '=' operator. + * It makes NR_CPUS size memcopy and bring memroy corruption. You have + * to use cpumask_copy() instead. */ #ifdef CONFIG_CPUMASK_OFFSTACK typedef struct cpumask *cpumask_var_t; -- 1.7.3.1
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