On 10/08/2015 05:25 PM, Andy Lutomirski wrote:
This whole isolation vs vmstat, etc thing made me think: It seems to me that a big part of the problem is that there's all kinds of per-cpu deferred housekeeping work that can be done on the CPU in question without any complicated or heavyweight locking but that can't be done remotely without a mess. This presumably includes vmstat, draining the LRU list, etc. This is a problem for allowing CPUs to spend a long time without any interrupts. I want to propose a new primitive that might go a long way toward solving this issue. The new primitive would be called the "big percpu lock". Non-nohz CPUs would hold their big percpu lock all the time. Nohz CPUs would hold it all the time unless idle. Full nohz cpus would hold it all the time except when idle or in user mode. No CPU promises to hold it while processing an NMI or similar NMI-like work. This should help in a ton of cases. For vunmap global kernel TLB flushes, we could stick the flushes in a list of deferred flushes to be processed on entry, and that list would be protected by the big percpu lock. For any kind of draining of non-NMI-safe percpu data (LRU, vmstat, whatever), we could have a housekeeping cpu try to do it using the big percpu lock There's a race here that affects task isolation. On exit to user mode, there's no obvious way to tell that an IPI is already pending. We could add that, too: whenever we send an IPI to a nohz_full CPU, we increment a percpu pending IPI count, then try to get the big percpu lock, and then, if we fail, send the IPI. IOW, we might want a helper that takes a remote big percpu lock or calls a remote function that guards against this race. Thoughts? Am I nuts?
The Tilera code has support for avoiding TLB flushes to kernel VAs while running in userspace on nohz_full cores, but I didn't try to upstream it yet because it is generally less critical than the other stuff. The model I chose is to have a per-cpu state that indicates whether the core is in kernel space, in user space, or in user space with a TLB flush pending. On entry to user space with task isolation in effect we just set the state to "user". When doing a remote TLB flush we decide whether or not to actually issue the flush by doing a cmpxchg() from "user" to "user pending", and if the old state was either "user" or "user pending", we don't issue the flush. Finally, on entry to the kernel for a task-isolation task we do an atomic xchg() to set the state to "kernel", and if we discover a flush was pending, we just globally flush the kernel's full VA range (no real reason to optimize for this case). This is basically equivalent to your lock model, where you would remotely trylock, and if you succeed, set a bit for the core indicating it needs a kernel TLB flush, and if you fail, just doing the remote flush yourself. And, on kernel entry for task isolation, you lock (possibly waiting while someone updates the kernel TLB flush state) and then if the kernel TLB flush bit is on, do the flush before completing the entry to the kernel. But, it turns out you also need to keep track of whether TLB flushes are currently pending for a given core, since you could start a remote TLB flush to a task-isolation core just as it was getting ready to return to userspace. Since the caller issues these flushes synchronously, we would just bump a counter atomically for the remote core before issuing, and decrement it when it was done. Then when returning to userspace, we first flip the bit saying that we are now in the "user" state, and then we actually spin and wait for the counter to hit zero as well, in case a TLB flush was in progress. For the tilegx architecture we had support for modifying how pages were statically cache homed, and this caused a lot of these TLB flushes since we needed to adjust the kernel linear mapping as well as the userspace mappings. It's a lot less common otherwise, just vunmap and the like, but still a good thing for a follow-up patch. -- Chris Metcalf, EZChip Semiconductor http://www.ezchip.com -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
