linux-kernel - Re: [PATCH linux-next] mm: swap: get rid of deadloop in swapin readahead

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Date:   Wed, 2 Mar 2022 12:38:00 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Michal Hocko <mhocko@...e.com>
cc:     Andrew Morton <akpm@...ux-foundation.org>, cgel.zte@...il.com,
        naoya.horiguchi@....com, minchan@...nel.org, hannes@...xchg.org,
        rogerq@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, guo.ziliang@....com.cn,
        Zeal Robot <zealci@....com.cn>,
        Ran Xiaokai <ran.xiaokai@....com.cn>,
        Jiang Xuexin <jiang.xuexin@....com.cn>,
        Yang Yang <yang.yang29@....com.cn>
Subject: Re: [PATCH linux-next] mm: swap: get rid of deadloop in swapin
 readahead

On Wed, 2 Mar 2022, Michal Hocko wrote:
> 
> I might be really missing something but I really do not see how is this
> any different from the page allocator path which only does cond_resched
> as well (well, except for throttling but that might just not trigger).
> Or other paths which just do cond_resched while waiting for a progress
> somewhere else.
> 
> Not that I like this situation but !PREEMPT kernel with RT priority
> tasks is rather limited and full of potential priblems IMHO.

As I said in previous mail, I have really not given this as much
thought this time as I did in the 2018 mail thread linked there;
but have seen that it behaves more badly than I had imagined, in
any preemptive kernel - no need for RT.  We just don't have the
stats to show when this code here spins waiting on code elsewhere
that is sleeping.  I think the difference from most cond_resched()
places is that swapin is trying to collect together several factors
with minimal locking, and we should have added preempt_disable()s
when preemption was invented.  But it's only swap so we didn't notice.

Hugh