On Tue, Oct 06, 2020 at 03:07:20PM +0100, Matthew Wilcox wrote:

On Mon, Oct 05, 2020 at 08:55:37PM -0700, Darrick J. Wong wrote:

quoted

On Mon, Oct 05, 2020 at 11:21:02AM -0400, Brian Foster wrote:

quoted

We've had reports of soft lockup warnings in the iomap ioend
completion path due to very large bios and/or bio chains. Divert any
ioends with 256k or more pages to process to the workqueue so
completion occurs in non-atomic context and can reschedule to avoid
soft lockup warnings.

Hmmmm... is there any way we can just make end_page_writeback faster?

There are ways to make it faster.  I don't know if they're a "just"
solution ...

1. We can use THPs.  That will reduce the number of pages being operated
on.  I hear somebody might have a patch set for that.  Incidentally,
this patch set will clash with the THP patchset, so one of us is going to
have to rebase on the other's work.  Not a complaint, just acknowledging
that some coordination will be needed for the 5.11 merge window.

How far off is this, anyway?  I assume it's in line behind the folio
series?

2. We could create end_writeback_pages(struct pagevec *pvec) which
calls a new test_clear_writeback_pages(pvec).  That could amortise
taking the memcg lock and finding the lruvec and taking the mapping
lock -- assuming these pages are sufficiently virtually contiguous.
It can definitely amortise all the statistics updates.

/me kinda wonders if THPs arent the better solution for people who want
to run large ios.

3. We can make wake_up_page(page, PG_writeback); more efficient.  If
you can produce this situation on demand, I had a patch for that which
languished due to lack of interest.

I can (well, someone can) so I'll talk to you internally about their
seeekret reproducer.

https://lore.kernel.org/linux-fsdevel/20200416220130.13343-1-willy@infradead.org/ (local)

--D