On Sun, Aug 20, 2023 at 02:59:07PM +0200, Linus Torvalds wrote: > On Sun, 20 Aug 2023 at 14:47, Linus Torvalds > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > But without that odd ifdef, I think it's fine. > > Another option might be to just move the might_sleep() to the top, and > do it unconditionally. If the trylock fails, the overhead of possibly > doing a cond_resched() is kind of moot. > I wanted to do it, but then I found this comment: * For example, if we have a kernel bug that causes a page * fault, we don't want to just use mmap_read_lock() to get * the mm lock, because that would deadlock if the bug were * to happen while we're holding the mm lock for writing. I figured scheduling away while on the way to OOPS/similar is not the best thing to happen. > IOW, the main problem here is not that it causes a scheduling point > (if the kernel isn't preemptable), it seems to be just that we > unnecessarily schedule in a place with the mm lock is held, so it > unnecessarily causes possible lock contention for writers. > > With the per-vma locking catching most cases, does any of this even matter? > > Mateusz - on that note: I'm wondering what made you see this as a > problem. The case you quote doesn't actually seem to be threaded, so > the vm lock contention shouldn't actually matter there. > > Does it schedule away? Sure. But only if "needs_resched" is set, so it > doesn't seem to be a *bad* thing per se. > > It might just be that this particular scheduling point ends up being a > common one on that load, and with those kernel config options (ie > PREEMPT_VOLUNTARY)? > I did not cause a slowdown for me and I did not say it did. I am saying down_read + going off CPU, should it happen in a multithreaded program, is going to delay any down_write issued by other threads. And that going off CPU here was clearly not intended. As I noted in another e-mail this is just a side thing I spotted while looking at other stuff. I don't find it important enough to discuss it any further, so as far as I am concerned you are most welcome to take any of the 2 patches, write your own or or leave the code be. [I am going to post other stuff later which *I am* going to argue to push for ;>]
