Messages in this thread | | | Date | Sat, 3 Apr 2021 01:04:35 +0000 | From | Fenghua Yu <> | Subject | Re: [PATCH v5 2/3] x86/bus_lock: Handle #DB for bus lock |
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Hi, Thomas,
On Sat, Mar 20, 2021 at 01:42:52PM +0100, Thomas Gleixner wrote: > On Fri, Mar 19 2021 at 22:19, Fenghua Yu wrote: > > On Fri, Mar 19, 2021 at 10:30:50PM +0100, Thomas Gleixner wrote: > >> > + if (sscanf(arg, "ratelimit:%d", &ratelimit) == 1 && ratelimit > 0) { > >> > + bld_ratelimit = ratelimit; > >> > >> So any rate up to INTMAX/s is valid here, right? > > > > Yes. I don't see smaller limitation than INTMX/s. Is that right? > > That's a given, but what's the point of limits in that range? > > A buslock access locks up the system for X cycles. So the total amount > of allowable damage in cycles per second is: > > limit * stall_cycles_per_bus_lock > > ergo the time (in seconds) which the system is locked up is: > > limit * stall_cycles_per_bus_lock / cpufreq > > Which means for ~INTMAX/2 on a 2 GHz CPU: > > 2 * 10^9 * $CYCLES / 2 * 10^9 = $CYCLES seconds > > Assumed the inflicted damage is only 1 cycle then #LOCK is pretty much > permanently on if there are enough threads. Sure #DB will slow them > down, but it still does not make any sense at all especially as the > damage is certainly greater than a single cycle. > > And because the changelogs and the docs are void of numbers I just got > real numbers myself. > > With a single thread doing a 'lock inc *mem' accross a cache line > boundary the workload which I measured with perf stat goes from: > > 5,940,985,091 instructions # 0.88 insn per cycle > 2.780950806 seconds time elapsed > 0.998480000 seconds user > 4.202137000 seconds sys > to > > 7,467,979,504 instructions # 0.10 insn per cycle > 5.110795917 seconds time elapsed > 7.123499000 seconds user > 37.266852000 seconds sys > > The buslock injection rate is ~250k per second. > > Even if I ratelimit the locked inc by a delay loop of ~5000 cycles > which is probably more than what the #DB will cost then this single task > still impacts the workload significantly: > > 6,496,994,537 instructions # 0.39 insn per cycle > 3.043275473 seconds time elapsed > 1.899852000 seconds user > 8.957088000 seconds sys > > The buslock injection rate is down to ~150k per second in this case. > > And even with throttling the injection rate further down to 25k per > second the impact on the workload is still significant in the 10% range.
Thank you for your insight!
So I can change the ratelimit to system wide and call usleep_range() to sleep: while (!__ratelimit(&global_bld_ratelimit)) usleep_range(1000000 / bld_ratelimit, 1000000 / bld_ratelimit);
The max bld_ratelimit is 1000,000/s because the max sleeping time is 1 usec. The min bld_ratelimit is 1/s.
> > And of course the documentation of the ratelimit parameter explains all > of this in great detail so the administrator has a trivial job to tune > that, right?
I will explain how to tune the parameter in buslock.rst doc.
> > >> > + case sld_ratelimit: > >> > + /* Enforce no more than bld_ratelimit bus locks/sec. */ > >> > + while (!__ratelimit(&get_current_user()->bld_ratelimit)) > >> > + msleep(1000 / bld_ratelimit); > > For any ratelimit > 1000 this will loop up to 1000 times with > CONFIG_HZ=1000. > > Assume that the buslock producer has tons of threads which all end up > here pretty soon then you launch a mass wakeup in the worst case every > jiffy. Are you sure that the cure is better than the disease?
if using usleep_range() to sleep, the threads will not sleep and wakeup, right? Maybe I can use msleep() for msec (bigger bld_ratelimit) and usleep_range() for usec (smaller bld_ratelimit)?
Even if there is mass wakeup, throttling the threads can avoid the system wide performance degradation (e.g. 7x slower dd command in another user). Is that a good justification for throttling the threads?
> > > If I split this whole patch set into two patch sets: > > 1. Three patches in the first patch set: the enumeration patch, the warn > > and fatal patch, and the documentation patch. > > 2. Two patches in the second patch set: the ratelimit patch and the > > documentation patch. > > > > Then I will send the two patch sets separately, you will accept them one > > by one. Is that OK? > > That's obviously the right thing to do because #1 should be ready and we > can sort out #2 seperately. See the conversation with Tony.
Thank you for picking up the first patch set!
-Fenghua
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