On 04/26/2016 04:50 PM, Mel Gorman wrote:
On Tue, Apr 26, 2016 at 03:41:22PM +0200, Vlastimil Babka wrote:
On 04/15/2016 11:07 AM, Mel Gorman wrote:
>There is a debugging check for callers that specify __GFP_DIRECT_RECLAIM
>from a context that cannot sleep. Triggering this is almost certainly
>a bug but it's also overhead in the fast path.
For CONFIG_DEBUG_ATOMIC_SLEEP, enabling is asking for the overhead. But for
CONFIG_PREEMPT_VOLUNTARY which turns it into _cond_resched(), I guess it's
not.
Either way, it struck me as odd. It does depend on the config and it's
marginal so if there is a problem then I can drop it.
What I tried to say is that it makes sense, but it's perhaps non-obvious :)
>Move the check to the slow
>path. It'll be harder to trigger as it'll only be checked when watermarks
>are depleted but it'll also only be checked in a path that can sleep.
Hmm what about zone_reclaim_mode=1, should the check be also duplicated to
that part of get_page_from_freelist()?
zone_reclaim has a !gfpflags_allow_blocking() check, does not call
cond_resched() before that check so it does not fall into an accidental
sleep path. I'm not seeing why the check is necessary there.
Hmm I thought the primary purpose of this might_sleep_if() is to catch those
(via the DEBUG_ATOMIC_SLEEP) that do pass __GFP_DIRECT_RECLAIM (which means
gfpflags_allow_blocking() will be true and zone_reclaim will proceed), but do so
from the wrong context. Am I getting that wrong?
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