Messages in this thread Patch in this message | | | Date | Tue, 29 Sep 2015 14:55:13 +0200 | From | Frederic Weisbecker <> | Subject | Re: [RFC][PATCH 02/11] sched: Create preempt_count invariant |
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On Tue, Sep 29, 2015 at 11:28:27AM +0200, Peter Zijlstra wrote: > Ensure that upon scheduling preempt_count == 2; although currently an > additional PREEMPT_ACTIVE is still possible. > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> > --- > arch/x86/include/asm/preempt.h | 3 ++- > include/asm-generic/preempt.h | 2 +- > kernel/sched/core.c | 14 ++++++++++---- > 3 files changed, 13 insertions(+), 6 deletions(-) > > --- a/arch/x86/include/asm/preempt.h > +++ b/arch/x86/include/asm/preempt.h > @@ -31,7 +31,8 @@ static __always_inline void preempt_coun > * must be macros to avoid header recursion hell > */ > #define init_task_preempt_count(p) do { \ > - task_thread_info(p)->saved_preempt_count = PREEMPT_DISABLED; \ > + task_thread_info(p)->saved_preempt_count = \ > + 2*PREEMPT_DISABLE_OFFSET + PREEMPT_NEED_RESCHED; \ > } while (0) > > #define init_idle_preempt_count(p, cpu) do { \ > --- a/include/asm-generic/preempt.h > +++ b/include/asm-generic/preempt.h > @@ -24,7 +24,7 @@ static __always_inline void preempt_coun > * must be macros to avoid header recursion hell > */ > #define init_task_preempt_count(p) do { \ > - task_thread_info(p)->preempt_count = PREEMPT_DISABLED; \ > + task_thread_info(p)->preempt_count = 2*PREEMPT_DISABLED; \
Since it's not quite obvious why we use this magic value without looking at schedule_tail() details, maybe add a little comment? (Just "/* see schedule_tail() */").
> } while (0) > > #define init_idle_preempt_count(p, cpu) do { \ > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -2588,11 +2588,17 @@ asmlinkage __visible void schedule_tail( > { > struct rq *rq; > > - /* finish_task_switch() drops rq->lock and enables preemtion */ > - preempt_disable(); > - rq = finish_task_switch(prev); > + /* > + * Still have preempt_count() == 2, from: > + * > + * schedule() > + * preempt_disable(); // 1 > + * __schedule() > + * raw_spin_lock_irq(&rq->lock) // 2 > + */
I found that a bit confusing first, because that's a preempt_count() we actually emulate for a new task. Maybe something like:
+ /* + * New task is init with preempt_count() == 2 because prev task left + * us after: + * + * schedule() + * preempt_disable(); // 1 + * __schedule() + * raw_spin_lock_irq(&rq->lock) // 2 + */
> + rq = finish_task_switch(prev); /* drops rq->lock, preempt_count() == 1 */ > balance_callback(rq); > - preempt_enable(); > + preempt_enable(); /* preempt_count() == 0 */
Thanks!
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