On Tue, Aug 11, 2015 at 2:47 PM, Paul E. McKenney <[email protected]> wrote: > On Tue, Aug 11, 2015 at 12:07:54PM -0700, Andy Lutomirski wrote: >> On Tue, Aug 11, 2015 at 11:33 AM, Paul E. McKenney >> <[email protected]> wrote: >> > On Tue, Aug 11, 2015 at 10:49:36AM -0700, Andy Lutomirski wrote: >> >> This is a bit late, but here goes anyway. >> >> >> >> Having played with the x86 context tracking hooks for awhile, I think >> >> it would be nice if core code that needs to be aware of CPU context >> >> (kernel, user, idle, guest, etc) could come up with single, >> >> comprehensible, easily validated set of hooks that arch code is >> >> supposed to call. >> >> >> >> Currently we have: >> >> >> >> - RCU hooks, which come in a wide variety to notify about IRQs, NMIs, >> >> etc. >> > >> > Something about people yelling at me for waking up idle CPUs, thus >> > degrading their battery lifetimes. ;-) >> > >> >> - Context tracking hooks. Only used by some arches. Calling these >> >> calls the RCU hooks for you in most cases. They have weird >> >> interactions with interrupts and they're slow. >> > >> > Combining these would be good, but there are subtleties. For example, >> > some arches don't have context tracking, but RCU still needs to correctly >> > identify idle CPUs without in any way interrupting or awakening that CPU. >> > It would be good to make this faster, but it does have to work. >> >> Could we maybe have one set of old RCU-only (no context tracking) >> callbacks and a completely separate set of callbacks for arches that >> support full context tracking? The implementation of the latter would >> presumably call into RCU. > > It should be possible for RCU to use context tracking if it is available > and to have RCU maintain its own state otherwise, if that is what you > are getting at. Assuming that the decision is global and made at either > build or boot time, anyway. Having some CPUs tracking context and others > not sounds like an invitation for subtle bugs.
I think that, if this happens, the decision should be made at build time, per arch, and not be configurable. If x86_64 uses context tracking, then I think x86_64 shouldn't need additional RCU callbacks, assuming that context tracking is comprehensive enough for RCU's purposes. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
