On Tuesday 17 March 2015 03:09 PM, Benjamin Herrenschmidt wrote: > On Tue, 2015-03-17 at 19:57 +1100, Michael Ellerman wrote: >> >> So my first preference is that you just bite the bullet and decide to either >> always apply the workaround, or just stick with the current behaviour. >> That's a >> trade-off between (I think) better idle latency but a risk of checkstops, vs. >> slower idle latency but less (how much less?) risk of checkstops. >> >> I think the reason you're proposing a kernel parameter is because we aren't >> willing to make that decision, ie. we're saying that users should decide. Is >> that right? > > Correct. More specifically, a fairly high profile user that I will not > name here has expressed interest in such a feature... > >> I'm not a big fan of kernel parameters. They are a pain to use, and are often >> just pushing a decision down one layer for no reason. What I mean is that >> individual users are probably just going to accept whatever the default value >> is from their distro. > > Right. This is quite an obscure tunable. > >> But anyway, that's a bit of a rant. >> >> As far as this patch is concerned, I don't think it actually needs to be a >> kernel parameter. >> >> >From what I can see below, the decision as to whether you apply the >> >workaround >> or not doesn't affect the list of idle states. So this could just as well be >> a >> runtime parameter, ie. a sysfs file, which can then be set by the user >> whenever >> they like? They might do it in a boot script, but that's up to them. > > Right, that would work too.
Okay. I'll send a patch with this design. > >> For simplicity I think it would also be fine to make it a write-once >> parameter, >> ie. you don't need to handle undoing it. > > It would be easy enough to make it rw using stop machine I think... > >> I think the only complication that would add is that you'd need to be a >> little >> careful about the order in which you nop out the calls vs applying the >> workaround, in case some threads are idle when you're called. Right, we should be safe with this sequence- - NOP call to undo workaround - Apply workaround on all cores. - NOP call to apply workaround > > I wouldn't bother with NOP'ing in that case, a runtime test will probably be > noise > in the measurement. > Didn't get your point here. Do you mean, ignore the request if some cores are in sleep or deeper state? > Cheers, > Ben. > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
