Re: [PATCH v1 1/1] s390/arch_random: Buffer true random data

Christian Borntraeger
2022-07-06T18:29:59Z

Am 06.07.22 um 18:26 schrieb Jason A. Donenfeld:
> Hi Harald,
>
> On Wed, Jul 06, 2022 at 06:18:27PM +0200, Harald Freudenberger wrote:
>> On 2022-07-05 18:27, Holger Dengler wrote:
>>> Hi Jason,
>>>
>>> On 05/07/2022 17:11, Jason A. Donenfeld wrote:
>>>> Hi Holger,
>>>>
>>>> On Tue, Jul 05, 2022 at 04:58:30PM +0200, Holger Dengler wrote:
>>>>> It is true, that the performance of the instruction is not really
>>>>> relevant, but only for calls outside of an interrupt context. I did
>>>>> some ftrace logging for the s390_random_get_seed_long() calls, and -
>>>>> as you said - there are a few calls per minute. But there was also
>>>>> some repeating calls in interrupt context. On systems with a huge
>>>>> interrupt load, this can cause severe performance impacts. I've no
>>>>
>>>> It'd be interesting to know more about this. The way you get
>>>> arch_random_get_seed_long() from irq context is:
>>>>
>>>> get_random_{bytes,int,long,u32,u64}()
>>>> crng_make_state()
>>>> crng_reseed() <-- Rarely
>>>> extract_entropy()
>>>> arch_get_random_seed_long()
>>>>
>>>> So if an irq user of get_random_xx() is the unlucky one in the minute
>>>> span who has to call crng_reseed() then, yea, that'll happen. But I
>>>> wonder about this luck aspect. What scenarios are you seeing where
>>>> this
>>>> happens all the time? Which driver is using random bytes *so* commonly
>>>> from irq context? Not that, per say, there's anything wrong with that,
>>>> but it could be eyebrow raising, and might point to de facto solutions
>>>> that mostly take care of this.
>>>
>>> I saw a few calls in interrupt context during my tracing, but I didn't
>>> look to see which ones they were. Let me figure that out in the next
>>> few days and provide more information on that.
>>>
>>>> One such direction might be making a driver that does such a thing do
>>>> it
>>>> a little bit less, somehow. Another direction would be preferring
>>>> non-irqs to handle crng_reseed(), but not disallowing irqs entirely,
>>>> with a patch something like the one below. Or maybe there are other
>>>> ideas.
>>>
>>> Reduce the number of trng in interrupt context is a possibility, but -
>>> in my opinion - only one single trng instruction call in interrupt
>>> context in one too much.
>>>
>>> For the moment, I would propose to drop the buffering but also return
>>> false, if arch_random_get_seed_long() is called in interrupt context.
>>>
>>> diff --git a/arch/s390/include/asm/archrandom.h
>>> b/arch/s390/include/asm/archrandom.h
>>> index 2c6e1c6ecbe7..711357bdc464 100644
>>> --- a/arch/s390/include/asm/archrandom.h
>>> +++ b/arch/s390/include/asm/archrandom.h
>>> @@ -32,7 +32,8 @@ static inline bool __must_check
>>> arch_get_random_int(unsigned int *v)
>>>
>>> static inline bool __must_check arch_get_random_seed_long(unsigned
>>> long *v)
>>> {
>>> - if (static_branch_likely(&s390_arch_random_available)) {
>>> + if (static_branch_likely(&s390_arch_random_available) &&
>>> + !in_interrupt()) {
>>> cpacf_trng(NULL, 0, (u8 *)v, sizeof(*v));
>>> atomic64_add(sizeof(*v), &s390_arch_random_counter);
>>> return true;
>>>
>>> (on-top of your commit, without our buffering patch)
>>>
>>>>
>>>> But all this is to say that having some more of the "mundane" details
>>>> about this might actually help us.
>>>>
>>>> Jason
>>>>
>>>> diff --git a/drivers/char/random.c b/drivers/char/random.c
>>>> index e3dd1dd3dd22..81df8cdf2a62 100644
>>>> --- a/drivers/char/random.c
>>>> +++ b/drivers/char/random.c
>>>> @@ -270,6 +270,9 @@ static bool crng_has_old_seed(void)
>>>> static bool early_boot = true;
>>>> unsigned long interval = CRNG_RESEED_INTERVAL;
>>>>
>>>> + if (in_hardirq())
>>>> + interval += HZ * 10;
>>>> +
>>>> if (unlikely(READ_ONCE(early_boot))) {
>>>> time64_t uptime = ktime_get_seconds();
>>>> if (uptime >= CRNG_RESEED_INTERVAL / HZ * 2)
>>>>
>>
>> Hi Holger and Jason
>> I tried to find out what is the reason of the invocations in interrupt
>> context.
>> First I have to admit that there is in fact not much of
>> arch_get_random_seed_long()
>> invocation any more in the recent kernel (5.19-rc5). I see about 100
>> invocations
>> within 10 minutes with an LPAR running some qperf and dd dumps on dasds
>> test load.
>> About half of these invocations is in interrupt context. I
>> dump_stack()ed some of
>> these and I always catch the function
>> kfence_guarded_alloc()
>> prandom_u32_max()
>> prandom_u32()
>> get_random_u32()
>> _get_random_bytes()
>> crng_make_state()
>> crng_reseed()
>> extract_entropy()
>> arch_get_random_seed_long()
>>
>> However, with so few invocations it should not make any harm when there
>> is a
>> even very expensive trng() invocation in interrupt context.
>>
>> But I think we should check, if this is really something to backport to
>> the older
>> kernels where arch_get_random_seed_long() is called really frequency.
>
> I backported the current random.c design to old kernels, so the
> situation there should be the same as in 5.19-rc5.
>
> So if you feel such rare usage is find even in_hardirq(), then I suppose
> there's nothing more to do here?

I think up to 190µs in interrupt can result in unwanted latencies. Yes it does not
happen that often and it is smaller than most timeslices of hypervisors.
So I would likely turn that question around
what happens if we return false if in_hardirq is true? Any noticeable
delays in code that uses random numbers?