linux-kernel - Re: [RFC PATCH] mm: silence soft lockups from unlock

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Date:   Fri, 7 Aug 2020 20:35:10 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [RFC PATCH] mm: silence soft lockups from unlock_page

On Fri, Aug 07, 2020 at 11:41:09AM -0700, Hugh Dickins wrote:
> My home testing was, effectively, on top of c6fe44d96fc1 (v5.8 plus
> your two patches): I did not have in the io_uring changes from the
> current tree. In glancing there, I noticed one new and one previous
> instance of SetPageWaiters() *after* __add_wait_queue_entry_tail():
> are those correct?

By the way, don't do any performance testing on current Linus tree.
This commit:

commit 37f4a24c2469a10a4c16c641671bd766e276cf9f (refs/bisect/bad)
Author: Ming Lei <ming.lei@...hat.com>
Date:   Tue Jun 30 22:03:57 2020 +0800

    blk-mq: centralise related handling into blk_mq_get_driver_tag
    
    Move .nr_active update and request assignment into blk_mq_get_driver_tag(),
    all are good to do during getting driver tag.
    
    Meantime blk-flush related code is simplified and flush request needn't
    to update the request table manually any more.
    
    Signed-off-by: Ming Lei <ming.lei@...hat.com>
    Cc: Christoph Hellwig <hch@...radead.org>
    Signed-off-by: Jens Axboe <axboe@...nel.dk>

slows everything down, at least on my test qemu system.  Seems like it's
losing block queue tags.