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. > 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. I wouldn't bother with NOP'ing in that case, a runtime test will probably be noise in the measurement. Cheers, Ben. _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@lists.ozlabs.org https://lists.ozlabs.org/listinfo/linuxppc-dev
