----- On Apr 2, 2018, at 11:33 AM, Mathieu Desnoyers mathieu.desnoyers@xxxxxxxxxxxx wrote: > ----- On Apr 1, 2018, at 12:13 PM, One Thousand Gnomes > gnomes@xxxxxxxxxxxxxxxxxxx wrote: > >> On Tue, 27 Mar 2018 12:05:23 -0400 >> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote: >> >>> Expose a new system call allowing each thread to register one userspace >>> memory area to be used as an ABI between kernel and user-space for two >>> purposes: user-space restartable sequences and quick access to read the >>> current CPU number value from user-space. >> >> What is the *worst* case timing achievable by using the atomics ? What >> does it do to real time performance requirements ? > > Given that there are two system calls introduced in this series (rseq and > cpu_opv), can you clarify which system call you refer to in the two questions > above ? > > For rseq, given that its userspace works pretty much like a read seqlock > (it retries on failure), it has no impact whatsoever on scheduler behavior. > So characterizing its worst case timing does not appear to be relevant. > >> For cpu_opv you now >> give an answer but your answer is assuming there isn't another thread >> actively thrashing the cache or store buffers, and that the user didn't >> sneakily pass in a page of uncacheable memory (eg framebuffer, or GPU >> space). > > Are those considered as device pages ? > >> >> I don't see anything that restricts it to cached pages. With that check >> in place for x86 at least it would probably be ok and I think the sneaky >> attacks to make it uncacheable would fail becuase you've got the pages >> locked so trying to give them to an accelerator will block until you are >> done. >> >> I still like the idea it's just the latencies concern me. > > Indeed, cpu_opv touches pages that are shared with user-space with > preemption off, so this one affects the scheduler latency. The worse-case > timings I measured for cpu_opv were with cache-cold memory. So I expect that > another thread actively trashing the cache would be in the same ballpark > figure. It does not account for a concurrent thread thrashing the store > buffers though. > > The checks enforcing which pages can be touched by cpu_opv operations are > done within cpu_op_check_page(). is_zone_device_page() is used to ensure no > device page is touched with preempt disabled. I understand that you would > prefer to disallow pages of uncacheable memory as well, which I'm fine with. > Is there an API similar to is_zone_device_page() to check whether a page is > uncacheable ? Looking into this a bit more, I notice the following: The pgprot_noncached (_PAGE_NOCACHE on x86) pgprot is part of the vma->vm_page_prot. Therefore, in order to have userspace provide pointers to noncached pages as input to cpu_opv, they need to be part of a userspace vma which has a pgprot_noncached vm_page_prot. The cpu_opv system call uses get_user_pages_fast() to grab the struct page from the userspace addresses, and then passes those pages to vm_map_ram(), with a PAGE_KERNEL pgprot. This creates a temporary kernel mapping to those pages, which is then used to read/write from/to those pages with preemption disabled. Therefore, with the proposed cpu_opv implementation, the kernel is not touching noncached mappings with preemption disabled, which should take care of your latency concern. Am I missing something ? Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
