Messages in this thread | | | Date | Tue, 16 Aug 2022 16:02:55 +0200 | From | "Jason A. Donenfeld" <> | Subject | Re: [PATCH] random: use raw spinlocks for use on RT |
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Hey Sebastian,
On Thu, Aug 11, 2022 at 04:20:21PM +0200, Jason A. Donenfeld wrote: > Hi Sebastian, > > On Thu, Aug 11, 2022 at 09:15:11AM +0200, Sebastian Andrzej Siewior wrote: > > On 2022-08-11 02:17:31 [+0200], Jason A. Donenfeld wrote: > > > Hey Sebastian, > > Hi Jason, > > > > > > > Sebastian - I won't move forward with this without your Ack, obviously. > > > > > What do you think of this general approach? -Jason > > > > > > > > I would need to do worst-case measurements and I've been looking at this > > > > just before writting the other email and there was a local_lock_t > > > > somewhere which needs also change… > > > > > > Did you ever come up some measurements here? It sure would be nice if I > > > could apply this, but obviously that's contingent on you saying it's > > > okay latency-wise on RT. > > > > No, I did not. But I've been thinking a little about it. The worst case > > latency is important now and later. > > Looking at it, all we need is one init in vsprintf at boot time and we > > are done. That is the third fallout that I am aware of since the rework > > of get_random_*(). > > We managed to get rid of all memory allocations (including GFP_ATOMIC) > > from preempt/IRQ-off section on PREEMPT_RT. Therefore I am not convinced > > to make all locks in random core a raw_spinlock_t just to make things > > work here as of now. > > By grouping everything into "the rework of get_random_*()", you miss > important subtleties, as I mentioned before. Importantly, in this case, > the issue we're facing has absolutely nothing at all to do with that, > but is rather entirely the result of removing the async notifier > mechanism in favor of doing things more directly, more straight > forwardly. So let's not muddle what we're discussing here. > > But more generally, the RNG is supposed to be usable from any context. > And adding wild workarounds, or worse, adding back complex async > notifier stuff, seems bad. So far your proposals for the printk issue > haven't been acceptable at all. > > So why don't we actually fix this, so we don't have to keep coming up > with hacks? The question is: does using raw spinlocks over this code > result in any real issue for RT latency? If so, I'd like to know where, > and maybe I can do something about that (or maybe I can't). If not, then > this is a non problem and I'll apply this patch with your blessing. > > If you don't want to spend time doing latency measurements, could you > instead share a document or similar to the type of methodology you > usually use for that, so I can do the same? And at the very least, I am > simply curious and want to know more about the RT world.
Thought I'd ping you about this again...
Jason
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