Messages in this thread | | | Date | Sun, 19 Jun 2011 23:38:19 -0300 | From | Glauber Costa <> | Subject | Re: [PATCH v2 5/7] KVM-GST: KVM Steal time accounting |
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On 06/19/2011 07:04 AM, Avi Kivity wrote: > On 06/17/2011 01:20 AM, Glauber Costa wrote: >> This patch accounts steal time time in kernel/sched. >> I kept it from last proposal, because I still see advantages >> in it: Doing it here will give us easier access from scheduler >> variables such as the cpu rq. The next patch shows an example of >> usage for it. >> >> Since functions like account_idle_time() can be called from >> multiple places, not only account_process_tick(), steal time >> grabbing is repeated in each account function separatedely. >> >> /* >> + * We have to at flush steal time information every time something else >> + * is accounted. Since the accounting functions are all visible to >> the rest >> + * of the kernel, it gets tricky to do them in one place. This helper >> function >> + * helps us. >> + * >> + * When the system is idle, the concept of steal time does not apply. >> We just >> + * tell the underlying hypervisor that we grabbed the data, but skip >> steal time >> + * accounting >> + */ >> +static inline bool touch_steal_time(int is_idle) >> +{ >> + u64 steal, st = 0; >> + >> + if (static_branch(¶virt_steal_enabled)) { >> + >> + steal = paravirt_steal_clock(smp_processor_id()); >> + >> + steal -= this_rq()->prev_steal_time; >> + if (is_idle) { >> + this_rq()->prev_steal_time += steal; >> + return false; >> + } >> + >> + while (steal>= TICK_NSEC) { >> + /* >> + * Inline assembly required to prevent the compiler >> + * optimising this loop into a divmod call. >> + * See __iter_div_u64_rem() for another example of this. >> + */ > > Why not use said function?
because here we want to do work during each loop. The said function would have to be adapted for that, possibly using a macro, to run arbitrary code during each loop iteration, in a way that I don't think it is worthy given the current number of callers (2 counting this new one)
>> + asm("" : "+rm" (steal)); >> + >> + steal -= TICK_NSEC; >> + this_rq()->prev_steal_time += TICK_NSEC; >> + st++; > > Suppose a live migration or SIGSTOP causes lots of steal time. How long > will we spend here? Silly me. I actually used this same argument with Peter to cap it with "delta" in the next patch in this series. So I think you are 100 % right. Here, however, we do want to account all that time, I believe.
How about we do a slow division if we're > 10 sec (unlikely), and account everything as steal time in this scenario ?
>> + } >> + >> + account_steal_time(st); >> + return !!st; > > !! !needed, you're returning a bool.
ah, sure thing.
>> + } >> + return false; >> +} >> + > > I'll need Peter's (or another sched maintainer's) review to apply this. >
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