> On Apr 19, 2023, at 6:15 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > > On Wed, Apr 19, 2023 at 08:14:09AM -0300, Marcelo Tosatti wrote: >> On Tue, Apr 18, 2023 at 03:02:00PM -0700, Andrew Morton wrote: >>> On Mon, 20 Mar 2023 15:03:32 -0300 Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: >>> >>>> This patch series addresses the following two problems: >>>> >>>> 1. A customer provided evidence indicating that a process >>>> was stalled in direct reclaim: >>>> >>>> ... >>>> >>>> 2. With a task that busy loops on a given CPU, >>>> the kworker interruption to execute vmstat_update >>>> is undesired and may exceed latency thresholds >>>> for certain applications. >>>> >>> >>> I don't think I'll be sending this upstream in the next merge window. >>> Because it isn't clear that the added complexity in vmstat handling is >>> justified. >> >> From my POV this is an incorrect statement (that the complexity in >> vmstat handling is not justified). >> >> Andrew, this is the 3rd attempt to fix this problem: >> >> First try: https://lore.kernel.org/lkml/20220127173037.318440631@fedora.localdomain/ >> >> Second try: https://patchew.org/linux/20230105125218.031928326@xxxxxxxxxx/ >> >> Third try: syncing vmstats remotely from vmstat_shepherd (this >> patchset). >> >> And also, can you please explain: what is so complicated about the >> vmstat handling? cmpxchg has been around and is used all over the >> kernel, and nobody considers "excessively complicated". >> >>> - Michal's request for more clarity on the end-user requirements >>> seems reasonable. >> >> And i explained to Michal in great detail where the end-user >> requirements come from. For virtualized workloads, there are two >> types of use-cases: >> >> 1) For example, for the MAC scheduler processing must occur every 1ms, >> and a certain amount of computation takes place (and must finish before >> the next 1ms timeframe). A > 50us latency spike as observed by cyclictest >> is considered a "failure". >> >> I showed him a 7us trace caused by, and explained that will extend to > >> 50us in the case of virtualized vCPU. >> >> 2) PLCs. These workloads will also suffer > 50us latency spikes >> which is undesirable. >> >> Can you please explain what additional clarity is required? >> >> RH's performance team, for example, has been performing packet >> latency tests and waiting for this issue to be fixed for about 2 >> years now. >> >> Andrew Theurer, can you please explain what problem is the vmstat_work >> interruption causing in your testing? > > +CC Andrew. Nearly every telco we work with for 5G RAN is demanding <20 usec CPU latency as measured by cyclictest & oslat. We cannot achieve under 20 usec with the vmstats interruption. -Andrew
