On Tue, 2014-10-28 at 09:25 -0700, Linus Torvalds wrote: > > Since we have hardware broadcasting of TLB invalidations on ARM, it is > > in our interest to keep the number of outstanding operations as small as > > possible, particularly on large systems where we don't get the targetted > > shootdown with a single message that you can perform using IPIs (i.e. > > you can only broadcast to all or no CPUs, and that happens for each pte). > > Do you seriously *have* to broadcast for each pte?
We do too, in current CPUs at least, it's sad ... > Because that is quite frankly moronic. We batch things up in software > for a real good reason: doing things one entry at a time just cannot > ever scale. At some point (and that point is usually not even very far > away), it's much better to do a single invalidate over a range. The > cost of having to refill the TLB's is *much* smaller than the cost of > doing tons of cross-CPU invalidates. > > That's true even for the cases where we track the CPU's involved in > that mapping, and only invalidate a small subset. With a "all CPU's > broadcast", the cross-over point must be even smaller. Doing thousands > of CPU broadcasts is just crazy, even if they are hw-accelerated. > > Can't you just do a full invalidate and a SW IPI for larger ranges? For us, this would be great except ... we can potentially have other agents with an MMU that only support snooping of the broadcasts... > And as mentioned, true sparse mappings are actually fairly rare, so > making extra effort (and data structures) to have individual ranges > sounds crazy. > > Is this some hw-enforced thing? You really can't turn off the > cross-cpu-for-each-pte braindamage? Cheers, Ben. > Linus > -- > 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/ -- 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/
