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

[Hugh Dickins <hughd@google.com>]

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