On Thu, 30 Mar 2017 06:46:30 +0800 Wanpeng Li <kernellwp@xxxxxxxxx> wrote: > > So! Now we need to find a proper fix :o) > > > > Hmm, how bad would it be to revert to sched_clock() instead of jiffies in vtime_delta()? > > We could use nanosecond granularity to check deltas but only perform an actual cputime update > > when that delta >= TICK_NSEC. That should keep the load ok. > > Yeah, I mentioned something similar before. > https://lkml.org/lkml/2017/3/26/138 However, Rik's commit optimized > syscalls by not utilize sched_clock(), so if we should distinguish > between syscalls/exceptions and irqs? Why not use ktime_get()? Here's the solution I was thinking about, it's mostly untested. I'm rate limiting below TICK_NSEC because I want to avoid syncing with the tick. diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c index f3778e2b..a8b1e85 100644 --- a/kernel/sched/cputime.c +++ b/kernel/sched/cputime.c @@ -676,18 +676,20 @@ void thread_group_cputime_adjusted(struct task_struct *p, u64 *ut, u64 *st) #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN static u64 vtime_delta(struct task_struct *tsk) { - unsigned long now = READ_ONCE(jiffies); + return ktime_sub(ktime_get(), tsk->vtime_snap); +} - if (time_before(now, (unsigned long)tsk->vtime_snap)) - return 0; +/* A little bit less than the tick period */ +#define VTIME_RATE_LIMIT (TICK_NSEC - 200000) - return jiffies_to_nsecs(now - tsk->vtime_snap); +static bool vtime_should_account(struct task_struct *tsk) +{ + return vtime_delta(tsk) > VTIME_RATE_LIMIT; } static u64 get_vtime_delta(struct task_struct *tsk) { - unsigned long now = READ_ONCE(jiffies); - u64 delta, other; + u64 delta, other, now = ktime_get(); /* * Unlike tick based timing, vtime based timing never has lost @@ -696,7 +698,7 @@ static u64 get_vtime_delta(struct task_struct *tsk) * elapsed time. Limit account_other_time to prevent rounding * errors from causing elapsed vtime to go negative. */ - delta = jiffies_to_nsecs(now - tsk->vtime_snap); + delta = ktime_sub(now, tsk->vtime_snap); other = account_other_time(delta); WARN_ON_ONCE(tsk->vtime_snap_whence == VTIME_INACTIVE); tsk->vtime_snap = now; @@ -711,7 +713,7 @@ static void __vtime_account_system(struct task_struct *tsk) void vtime_account_system(struct task_struct *tsk) { - if (!vtime_delta(tsk)) + if (!vtime_should_account(tsk)) return; write_seqcount_begin(&tsk->vtime_seqcount); @@ -723,7 +725,7 @@ void vtime_account_user(struct task_struct *tsk) { write_seqcount_begin(&tsk->vtime_seqcount); tsk->vtime_snap_whence = VTIME_SYS; - if (vtime_delta(tsk)) + if (vtime_should_account(tsk)) account_user_time(tsk, get_vtime_delta(tsk)); write_seqcount_end(&tsk->vtime_seqcount); } @@ -731,7 +733,7 @@ void vtime_account_user(struct task_struct *tsk) void vtime_user_enter(struct task_struct *tsk) { write_seqcount_begin(&tsk->vtime_seqcount); - if (vtime_delta(tsk)) + if (vtime_should_account(tsk)) __vtime_account_system(tsk); tsk->vtime_snap_whence = VTIME_USER; write_seqcount_end(&tsk->vtime_seqcount); @@ -747,7 +749,7 @@ void vtime_guest_enter(struct task_struct *tsk) * that can thus safely catch up with a tickless delta. */ write_seqcount_begin(&tsk->vtime_seqcount); - if (vtime_delta(tsk)) + if (vtime_should_account(tsk)) __vtime_account_system(tsk); current->flags |= PF_VCPU; write_seqcount_end(&tsk->vtime_seqcount); @@ -776,7 +778,7 @@ void arch_vtime_task_switch(struct task_struct *prev) write_seqcount_begin(¤t->vtime_seqcount); current->vtime_snap_whence = VTIME_SYS; - current->vtime_snap = jiffies; + current->vtime_snap = ktime_get(); write_seqcount_end(¤t->vtime_seqcount); } -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
