Re: [PATCH v5 2/3] x86/bus_lock: Handle #DB for bus lock

[Thomas Gleixner]

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.

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?

>> > +	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 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.

Thanks,

        tglx