shrink_page_list() only applies if inactive pages were isolated
which in turn may not happen if all_unreclaimable is set in
shrink_zones(). If for whatver reason, all_unreclaimable is
set on all zones, we can miss calling cond_resched().
shrink_slab only applies if we are reclaiming slab pages. If the first
shrinker returns -1, we do not call cond_resched(). If that
first shrinker is dcache and __GFP_FS is not set, direct
reclaimers will not shrink at all. However, if there are
enough of them running or if one of the other shrinkers
is running for a very long time, kswapd could be starved
acquiring the shrinker_rwsem and never reaching the
balance_pgdat() only calls cond_resched if the zones are not
balanced. For a high-order allocation that is balanced, it
checks order-0 again. During that window, order-0 might have
become unbalanced so it loops again for order-0 and returns
that was reclaiming for order-0 to kswapd(). It can then find
that a caller has rewoken kswapd for a high-order and re-enters
balance_pgdat() without ever have called cond_resched().
While it appears unlikely, there are bad conditions which can result
in cond_resched() being avoided.