On Wed, 19 Aug 2020 14:28:49 -0400 Robert Foley <robert.fo...@linaro.org> wrote:
> The purpose of this change is to set the groundwork > so that an arch could move towards removing > the BQL from the cpu_handle_interrupt/exception paths. > > The BQL is a bottleneck in scaling to more cores. > And this cpu_handle_interrupt/exception path is one of > the key BQL users as measured by the QEMU sync profiling (qsp). > > As the first step in removing the BQL from this path, we will make > changes to the core/common functions of cpu_handle_interrupt/exception > to drop the holding of the BQL. The holding of the BQL is pushed down > to the per-arch implementation code. I have only skimmed the patches I was cc:ed on so far, but the series seems sane to me in general. > > This patch goes through several transitions of the code in order to > maintain correctness (bisectability). In order to maintain > bisectability across these steps some patches need to touch many > files across different arches, however most of the changes are trivial. > > The general order of the changes is below where each step > represents one patch. > > 1) rename all *_do_interrupt functions to *_do_interrupt_locked I'm wondering whether this renaming could be done in an automated way (e.g. via Coccinelle). Reviewing the method for the renaming is often easier than looking at a lot of similar code patterns. > > 2) add a new function *_do_interrupt that takes the BQL and calls > *_do_interrupt_locked, point ->do_interrupt to it, and remove > the BQL from cpu-exec.c's cpu_handle_exception. > > 3) modify the BQL critical sections around > ->cpu_exec_interrupt, so that the BQL critical section covers just the > call to ->cpu_exec_interrupt. > > 4/5) same as 1/2 for ->cpu_exec_interrupt. This removes the BQL > from cpu_handle_exception. The method of doing this in steps looks fine, although the patches produced are a bit unwieldy -- that's something we have to live with, I guess.
