From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <541B3303.9060003@xenomai.org> Date: Thu, 18 Sep 2014 21:31:15 +0200 From: Gilles Chanteperdrix MIME-Version: 1.0 References: <535828F6.6050308@xenomai.org> <53583DF7.3080700@xenomai.org> <540F6B15.2070201@xenomai.org> <54112EFA.4080901@web.de> <541130D0.50409@web.de> <541AC62F.2050003@xenomai.org> <541AC933.9090600@siemens.com> <541ACD66.50902@xenomai.org> <541ACE05.1050305@siemens.com> <541AD8A2.30500@xenomai.org> <541ADD8A.7020804@siemens.com> <541AE1CD.7090008@xenomai.org> <541B04E6.5010808@siemens.com> <541B0815.5010906@xenomai.org> <541B26FE.6000507@xenomai.org> <541B311B.1060501@siemens.com> In-Reply-To: <541B311B.1060501@siemens.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai] Reading /proc/xenomai/stat causes high latencies List-Id: Discussions about the Xenomai project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka , Jeroen Van den Keybus Cc: "xenomai@xenomai.org" On 09/18/2014 09:23 PM, Jan Kiszka wrote: > On 2014-09-18 20:39, Gilles Chanteperdrix wrote: >> On 09/18/2014 06:28 PM, Gilles Chanteperdrix wrote: >>> On 09/18/2014 06:14 PM, Jan Kiszka wrote: >>>> On 2014-09-18 15:44, Gilles Chanteperdrix wrote: >>>>> On 09/18/2014 03:26 PM, Jan Kiszka wrote: >>>>>> On 2014-09-18 15:05, Gilles Chanteperdrix wrote: >>>>>>> On 09/18/2014 02:20 PM, Jan Kiszka wrote: >>>>>>>> On 2014-09-18 14:17, Gilles Chanteperdrix wrote: >>>>>>>>> On 09/18/2014 01:59 PM, Jan Kiszka wrote: >>>>>>>>>> On 2014-09-18 13:46, Gilles Chanteperdrix wrote: >>>>>>>>>>> On 09/11/2014 07:19 AM, Jan Kiszka wrote: >>>>>>>>>>>> On 2014-09-11 07:11, Jan Kiszka wrote: >>>>>>>>>>>>> On 2014-09-09 23:03, Gilles Chanteperdrix wrote: >>>>>>>>>>>>>> On 04/25/2014 12:44 PM, Jeroen Van den Keybus wrote: >>>>>>>>>>>>>>> For testing, I've removed the locks from the vfile system. >>>>>>>>>>>>>>> Then the high latencies reliably disappear. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> To test, I made two xeno_nucleus modules: one with the >>>>>>>>>>>>>>> xnlock_get/put_ in place and one with dummies. Subsequently, >>>>>>>>>>>>>>> I use a program that simply opens and reads the stat file >>>>>>>>>>>>>>> 1,000 times. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> With locks: >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> RTT| 00:00:01 (periodic user-mode task, 100 us period, >>>>>>>>>>>>>>> priority 99) RTH|----lat min|----lat avg|----lat >>>>>>>>>>>>>>> max|-overrun|---msw|---lat best|--lat worst RTD| -2.575| >>>>>>>>>>>>>>> -2.309| 9.286| 0| 0| -2.575| 9.286 >>>>>>>>>>>>>>> RTD| -2.364| -2.276| 1.600| 0| 0| >>>>>>>>>>>>>>> -2.575| 9.286 RTD| -2.482| -2.274| 2.165| >>>>>>>>>>>>>>> 0| 0| -2.575| 9.286 RTD| -2.368| 135.261| >>>>>>>>>>>>>>> 1478.154| 13008| 0| -2.575| 1478.154 RTD| >>>>>>>>>>>>>>> -2.368| -2.272| 2.602| 13008| 0| -2.575| >>>>>>>>>>>>>>> 1478.154 RTD| -2.499| -2.272| 6.933| 13008| >>>>>>>>>>>>>>> 0| -2.575| 1478.154 >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Without locks: >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> RTT| 00:00:01 (periodic user-mode task, 100 us period, >>>>>>>>>>>>>>> priority 99) RTH|----lat min|----lat avg|----lat >>>>>>>>>>>>>>> max|-overrun|---msw|---lat best|--lat worst RTD| -2.503| >>>>>>>>>>>>>>> -2.270| 3.310| 0| 0| -2.503| 3.310 >>>>>>>>>>>>>>> RTD| -2.418| -2.284| -1.646| 0| 0| >>>>>>>>>>>>>>> -2.503| 3.310 RTD| -2.496| -2.275| 4.630| >>>>>>>>>>>>>>> 0| 0| -2.503| 4.630 RTD| -2.374| -2.285| >>>>>>>>>>>>>>> -1.458| 0| 0| -2.503| 4.630 RTD| >>>>>>>>>>>>>>> -2.452| -2.273| 3.559| 0| 0| -2.503| >>>>>>>>>>>>>>> 4.630 RTD| -2.370| -2.285| -1.518| 0| >>>>>>>>>>>>>>> 0| -2.503| 4.630 RTD| -2.458| -2.274| >>>>>>>>>>>>>>> 4.203| 0| 0| -2.503| 4.630 >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> I'll now have a closer look into the vfile system but if the >>>>>>>>>>>>>>> locks are malfunctioning, I'm clueless. >>>>>>>>>>>>>> >>>>>>>>>>>>>> Answering with a "little" delay, could you try the following >>>>>>>>>>>>>> patch? >>>>>>>>>>>>>> >>>>>>>>>>>>>> diff --git a/include/asm-generic/bits/pod.h >>>>>>>>>>>>>> b/include/asm-generic/bits/pod.h index a6be0dc..cfb0c71 100644 >>>>>>>>>>>>>> --- a/include/asm-generic/bits/pod.h +++ >>>>>>>>>>>>>> b/include/asm-generic/bits/pod.h @@ -248,6 +248,7 @@ void >>>>>>>>>>>>>> __xnlock_spin(xnlock_t *lock /*, */ XNLOCK_DBG_CONTEXT_ARGS) >>>>>>>>>>>>>> cpu_relax(); xnlock_dbg_spinning(lock, cpu, &spin_limit /*, */ >>>>>>>>>>>>>> XNLOCK_DBG_PASS_CONTEXT); + xnarch_memory_barrier(); } >>>>>>>>>>>>>> while(atomic_read(&lock->owner) != ~0); } >>>>>>>>>>>>>> EXPORT_SYMBOL_GPL(__xnlock_spin); diff --git >>>>>>>>>>>>>> a/include/asm-generic/system.h b/include/asm-generic/system.h >>>>>>>>>>>>>> index 25bd83f..7a8c4d0 100644 --- >>>>>>>>>>>>>> a/include/asm-generic/system.h +++ >>>>>>>>>>>>>> b/include/asm-generic/system.h @@ -378,6 +378,8 @@ static >>>>>>>>>>>>>> inline void xnlock_put(xnlock_t *lock) >>>>>>>>>>>>>> xnarch_memory_barrier(); >>>>>>>>>>>>>> >>>>>>>>>>>>>> atomic_set(&lock->owner, ~0); + + xnarch_memory_barrier(); >>>>>>>>>>>>> >>>>>>>>>>>>> That's pretty heavy-weighted now (it was already due to the first >>>>>>>>>>>>> memory barrier). Maybe it's better to look at some ticket lock >>>>>>>>>>>>> mechanism like Linux uses for fairness. At least on x86 (and >>>>>>>>>>>>> other strictly ordered archs), those require no memory barriers >>>>>>>>>>>>> on release. >>>>>>>>>>> >>>>>>>>>>>> In fact, memory barriers aren't needed on strictly ordered archs >>>>>>>>>>>> already today, independent of the spinlock granting algorithm. So >>>>>>>>>>>> there are two optimization possibilities: >>>>>>>>>>> >>>>>>>>>>>> - ticket-based granting - arch-specific (thus optimized) core >>>>>>>>>>> >>>>>>>>>>> Ok, no answer, so I will try to be more clear. >>>>>>>>>>> >>>>>>>>>>> I do not pretend to understand how memory barriers work at a low >>>>>>>>>>> level, this is a shame, I know, and am sorry for that. My "high level" >>>>>>>>>>> view, is that memory barriers on SMP systems act as synchronization >>>>>>>>>>> points, meaning that when a CPU issues a barrier, it will "see" the >>>>>>>>>>> state of the other CPUs at the time of their last barrier. This means >>>>>>>>>>> that for a CPU to see a store that occured on another CPU, there must >>>>>>>>>>> have been two barriers: a barrier after the store on one cpu, and a >>>>>>>>>>> barrier after that before the read on the other cpu. This view of >>>>>>>>>>> things seems to be corroborated by the fact that the patch works, and >>>>>>>>>>> by the following sentence in Documentation/memory-barriers.txt: >>>>>>>>>>> >>>>>>>>>>> (*) There is no guarantee that a CPU will see the correct order of >>>>>>>>>>> effects from a second CPU's accesses, even _if_ the second CPU uses a >>>>>>>>>>> memory barrier, unless the first CPU _also_ uses a matching memory >>>>>>>>>>> barrier (see the subsection on "SMP Barrier Pairing"). >>>>>>>>>> >>>>>>>>>> [quick answer] >>>>>>>>>> >>>>>>>>>> ...or the architecture refrains from reordering write requests, like x86 >>>>>>>>>> does. What may happen, though, is that the compiler reorders the writes. >>>>>>>>>> Therefore you need at least a (must cheaper) compiler barrier on those >>>>>>>>>> archs. See also linux/Documentation/memory-barriers.txt on this and more. >>>>>>>>> >>>>>>>>> quick answer: I do not believe an SMP architecture can enforce stores >>>>>>>>> ordering accross multiple cpus, with cpus local caches and such. And the >>>>>>>>> fact that the patch I sent fixed the issue on x86 tend to prove me right. >>>>>>>> >>>>>>>> It's not wrong, it's just (costly, on larger machines) overkill as the >>>>>>>> other cores either see the lock release and all prior changes committed >>>>>>>> or the lock taken (and the prior changes do not matter then). They will >>>>>>>> never see later changes committed before the lock being visible as free. >>>>>>> >>>>>>> I agree. But this is true on all architectures, not just on strictly >>>>>>> ordered ones, this is just due to how barriers work on SMP systems, as I >>>>>>> explained. >>>>>>> >>>>>>>> That's architecturally guaranteed, and that's why you have no memory >>>>>>>> barriers in x86 spinlock release operations. >>>>>>> >>>>>>> I disagree, as explained in the paragraph just below the one you quote, >>>>>>> I believe this is an optimization, which is almost valid on any >>>>>>> architecture. Almost valid, because if the cpu which has done the unlock >>>>>>> does another lock without any time for a barrier in between to >>>>>>> synchronize cpus, we have a problem, because the other cpus will never >>>>>>> see the spinlock as free. With ticket spinlocks, you just add a store on >>>>>>> the cpu which spins, and you have to add a barrier after that, if you >>>>>>> want the barrier before the read on the cpu which will acquire the lock >>>>>>> to see that the spinlock is contended. So I do not see how this requires >>>>>>> less barriers. >>>>>> >>>>>> Ticket locks prevent unfair starvation without the closing barrier as >>>>>> they grant the next ticket to the next waiter, not the current holder. >>>>>> See the Linux implementation. >>>>> >>>>> Whether to put the closing barrier after the last store is orthogonal, >>>>> to whether implementing ticket locks or not. This is all a question of >>>>> tradeoffs. >>>>> >>>>> Without the barrier after the last store, you increase the spinning time >>>>> due to time taken for the store to be visible on other cpus, but you >>>>> optimize the overhead of unlocking. >>>>> >>>>> With ticket spinlocks you avoid the starvation situation, at the expense >>>>> of increasing the overhread of spinlock operations. >>>>> >>>>> I do not know which is worse. I suspect all this does not make much of a >>>>> difference, and what dominates is the duration of spinlock sections anyway. >>>> >>>> I think the way classic Linux spinlock did this on x86 provide the answer. >>> >>> The situation is completely different: linux spinlocks are well split, >>> xenomai basically has one only spinlock, so chances are that it will be >>> more contended, so the heavy unlock path (the one which implements the >>> ticket stuff) will be triggered more often. Also, xenomai spinlock (we >>> can loose the s anyway) being more contended, the "pending store >>> barrier" optimization has in fact chances of being detrimental. And >>> finally, due to the way spinlocks are split, Linux has scalability >>> issues that Xenomai can not even begin to imagine tackling. >> >> Finally, in the eternal worst case vs average case fight, the worst case >> worth optimizing is the contended case in our case, and I believe adding >> the barrier after atomic_set in xnlock_put is what optimizes this worst >> case best, because, again, it reduces the time between the unlock and >> its visibility on the spinning cpu. That is at least something Linux >> does not have to care about, because the worst case is not what it is >> optimized for. > > Maybe. Unsure right now, if we see prolonged spinning time due to this > on x86. I suspect not as spinning is not only increasing latencies but > also burning CPU power uselessly, and that would be noticed and disliked > under Linux. Probably it does not have this issue, because it uses atomic add or atomic cmpxchg to unlock the spinlock as far as I can tell. But these instructions look as heavy as a barrier to me. -- Gilles.