On Tue, Nov 07, 2023 at 03:08:07PM -0800, Ankur Arora wrote: > There are broadly three sets of uses of cond_resched(): > > 1. Calls to cond_resched() out of the goodness of our heart, > otherwise known as avoiding lockup splats. > > 2. Open coded variants of cond_resched_lock() which call > cond_resched(). > > 3. Retry or error handling loops, where cond_resched() is used as a > quick alternative to spinning in a tight-loop. > > When running under a full preemption model, the cond_resched() reduces > to a NOP (not even a barrier) so removing it obviously cannot matter. > > But considering only voluntary preemption models (for say code that > has been mostly tested under those), for set-1 and set-2 the > scheduler can now preempt kernel tasks running beyond their time > quanta anywhere they are preemptible() [1]. Which removes any need > for these explicitly placed scheduling points. > > The cond_resched() calls in set-3 are a little more difficult. > To start with, given it's NOP character under full preemption, it > never actually saved us from a tight loop. > With voluntary preemption, it's not a NOP, but it might as well be -- > for most workloads the scheduler does not have an interminable supply > of runnable tasks on the runqueue. > > So, cond_resched() is useful to not get softlockup splats, but not > terribly good for error handling. Ideally, these should be replaced > with some kind of timed or event wait. > For now we use cond_resched_stall(), which tries to schedule if > possible, and executes a cpu_relax() if not. > > Almost all the cond_resched() calls are from set-1. Remove them. FOr the memcpy_kunit.c cases, I don't think there are preemption locations in its loops. Perhaps I'm misunderstanding something? Why will the memcpy test no longer produce softlockup splats? -Kees > > [1] https://lore.kernel.org/lkml/20231107215742.363031-1-ankur.a.arora@xxxxxxxxxx/ > > Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> > Cc: "David S. Miller" <davem@xxxxxxxxxxxxx> > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > Cc: Eric Dumazet <edumazet@xxxxxxxxxx> > Cc: Jakub Kicinski <kuba@xxxxxxxxxx> > Cc: Paolo Abeni <pabeni@xxxxxxxxxx> > Cc: Thomas Graf <tgraf@xxxxxxx> > Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx> > --- > lib/crc32test.c | 2 -- > lib/crypto/mpi/mpi-pow.c | 1 - > lib/memcpy_kunit.c | 5 ----- > lib/random32.c | 1 - > lib/rhashtable.c | 2 -- > lib/test_bpf.c | 3 --- > lib/test_lockup.c | 2 +- > lib/test_maple_tree.c | 8 -------- > lib/test_rhashtable.c | 10 ---------- > 9 files changed, 1 insertion(+), 33 deletions(-) > > diff --git a/lib/crc32test.c b/lib/crc32test.c > index 9b4af79412c4..3eee90482e9a 100644 > --- a/lib/crc32test.c > +++ b/lib/crc32test.c > @@ -729,7 +729,6 @@ static int __init crc32c_combine_test(void) > crc_full == test[i].crc32c_le)) > errors++; > runs++; > - cond_resched(); > } > } > > @@ -817,7 +816,6 @@ static int __init crc32_combine_test(void) > crc_full == test[i].crc_le)) > errors++; > runs++; > - cond_resched(); > } > } > > diff --git a/lib/crypto/mpi/mpi-pow.c b/lib/crypto/mpi/mpi-pow.c > index 2fd7a46d55ec..074534900b7e 100644 > --- a/lib/crypto/mpi/mpi-pow.c > +++ b/lib/crypto/mpi/mpi-pow.c > @@ -242,7 +242,6 @@ int mpi_powm(MPI res, MPI base, MPI exp, MPI mod) > } > e <<= 1; > c--; > - cond_resched(); > } > > i--; > diff --git a/lib/memcpy_kunit.c b/lib/memcpy_kunit.c > index 440aee705ccc..c2a6b09fe93a 100644 > --- a/lib/memcpy_kunit.c > +++ b/lib/memcpy_kunit.c > @@ -361,8 +361,6 @@ static void copy_large_test(struct kunit *test, bool use_memmove) > /* Zero out what we copied for the next cycle. */ > memset(large_dst + offset, 0, bytes); > } > - /* Avoid stall warnings if this loop gets slow. */ > - cond_resched(); > } > } > > @@ -489,9 +487,6 @@ static void memmove_overlap_test(struct kunit *test) > for (int s_off = s_start; s_off < s_end; > s_off = next_step(s_off, s_start, s_end, window_step)) > inner_loop(test, bytes, d_off, s_off); > - > - /* Avoid stall warnings. */ > - cond_resched(); > } > } > } > diff --git a/lib/random32.c b/lib/random32.c > index 32060b852668..10bc804d99d6 100644 > --- a/lib/random32.c > +++ b/lib/random32.c > @@ -287,7 +287,6 @@ static int __init prandom_state_selftest(void) > errors++; > > runs++; > - cond_resched(); > } > > if (errors) > diff --git a/lib/rhashtable.c b/lib/rhashtable.c > index 6ae2ba8e06a2..5ff0f521bf29 100644 > --- a/lib/rhashtable.c > +++ b/lib/rhashtable.c > @@ -328,7 +328,6 @@ static int rhashtable_rehash_table(struct rhashtable *ht) > err = rhashtable_rehash_chain(ht, old_hash); > if (err) > return err; > - cond_resched(); > } > > /* Publish the new table pointer. */ > @@ -1147,7 +1146,6 @@ void rhashtable_free_and_destroy(struct rhashtable *ht, > for (i = 0; i < tbl->size; i++) { > struct rhash_head *pos, *next; > > - cond_resched(); > for (pos = rht_ptr_exclusive(rht_bucket(tbl, i)), > next = !rht_is_a_nulls(pos) ? > rht_dereference(pos->next, ht) : NULL; > diff --git a/lib/test_bpf.c b/lib/test_bpf.c > index ecde4216201e..15b4d32712d8 100644 > --- a/lib/test_bpf.c > +++ b/lib/test_bpf.c > @@ -14758,7 +14758,6 @@ static __init int test_skb_segment(void) > for (i = 0; i < ARRAY_SIZE(skb_segment_tests); i++) { > const struct skb_segment_test *test = &skb_segment_tests[i]; > > - cond_resched(); > if (exclude_test(i)) > continue; > > @@ -14787,7 +14786,6 @@ static __init int test_bpf(void) > struct bpf_prog *fp; > int err; > > - cond_resched(); > if (exclude_test(i)) > continue; > > @@ -15171,7 +15169,6 @@ static __init int test_tail_calls(struct bpf_array *progs) > u64 duration; > int ret; > > - cond_resched(); > if (exclude_test(i)) > continue; > > diff --git a/lib/test_lockup.c b/lib/test_lockup.c > index c3fd87d6c2dd..9af5d34c98f6 100644 > --- a/lib/test_lockup.c > +++ b/lib/test_lockup.c > @@ -381,7 +381,7 @@ static void test_lockup(bool master) > touch_nmi_watchdog(); > > if (call_cond_resched) > - cond_resched(); > + cond_resched_stall(); > > test_wait(cooldown_secs, cooldown_nsecs); > > diff --git a/lib/test_maple_tree.c b/lib/test_maple_tree.c > index 464eeb90d5ad..321fd5d8aef3 100644 > --- a/lib/test_maple_tree.c > +++ b/lib/test_maple_tree.c > @@ -2672,7 +2672,6 @@ static noinline void __init check_dup(struct maple_tree *mt) > rcu_barrier(); > } > > - cond_resched(); > mt_cache_shrink(); > /* Check with a value at zero, no gap */ > for (i = 1000; i < 2000; i++) { > @@ -2682,7 +2681,6 @@ static noinline void __init check_dup(struct maple_tree *mt) > rcu_barrier(); > } > > - cond_resched(); > mt_cache_shrink(); > /* Check with a value at zero and unreasonably large */ > for (i = big_start; i < big_start + 10; i++) { > @@ -2692,7 +2690,6 @@ static noinline void __init check_dup(struct maple_tree *mt) > rcu_barrier(); > } > > - cond_resched(); > mt_cache_shrink(); > /* Small to medium size not starting at zero*/ > for (i = 200; i < 1000; i++) { > @@ -2702,7 +2699,6 @@ static noinline void __init check_dup(struct maple_tree *mt) > rcu_barrier(); > } > > - cond_resched(); > mt_cache_shrink(); > /* Unreasonably large not starting at zero*/ > for (i = big_start; i < big_start + 10; i++) { > @@ -2710,7 +2706,6 @@ static noinline void __init check_dup(struct maple_tree *mt) > check_dup_gaps(mt, i, false, 5); > mtree_destroy(mt); > rcu_barrier(); > - cond_resched(); > mt_cache_shrink(); > } > > @@ -2720,7 +2715,6 @@ static noinline void __init check_dup(struct maple_tree *mt) > check_dup_gaps(mt, i, false, 5); > mtree_destroy(mt); > rcu_barrier(); > - cond_resched(); > if (i % 2 == 0) > mt_cache_shrink(); > } > @@ -2732,7 +2726,6 @@ static noinline void __init check_dup(struct maple_tree *mt) > check_dup_gaps(mt, i, true, 5); > mtree_destroy(mt); > rcu_barrier(); > - cond_resched(); > } > > mt_cache_shrink(); > @@ -2743,7 +2736,6 @@ static noinline void __init check_dup(struct maple_tree *mt) > mtree_destroy(mt); > rcu_barrier(); > mt_cache_shrink(); > - cond_resched(); > } > } > > diff --git a/lib/test_rhashtable.c b/lib/test_rhashtable.c > index c20f6cb4bf55..e5d1f272f2c6 100644 > --- a/lib/test_rhashtable.c > +++ b/lib/test_rhashtable.c > @@ -119,7 +119,6 @@ static int insert_retry(struct rhashtable *ht, struct test_obj *obj, > > do { > retries++; > - cond_resched(); > err = rhashtable_insert_fast(ht, &obj->node, params); > if (err == -ENOMEM && enomem_retry) { > enomem_retries++; > @@ -253,8 +252,6 @@ static s64 __init test_rhashtable(struct rhashtable *ht, struct test_obj *array, > > rhashtable_remove_fast(ht, &obj->node, test_rht_params); > } > - > - cond_resched(); > } > > end = ktime_get_ns(); > @@ -371,8 +368,6 @@ static int __init test_rhltable(unsigned int entries) > u32 i = get_random_u32_below(entries); > u32 prand = get_random_u32_below(4); > > - cond_resched(); > - > err = rhltable_remove(&rhlt, &rhl_test_objects[i].list_node, test_rht_params); > if (test_bit(i, obj_in_table)) { > clear_bit(i, obj_in_table); > @@ -412,7 +407,6 @@ static int __init test_rhltable(unsigned int entries) > } > > for (i = 0; i < entries; i++) { > - cond_resched(); > err = rhltable_remove(&rhlt, &rhl_test_objects[i].list_node, test_rht_params); > if (test_bit(i, obj_in_table)) { > if (WARN(err, "cannot remove element at slot %d", i)) > @@ -607,8 +601,6 @@ static int thread_lookup_test(struct thread_data *tdata) > obj->value.tid, obj->value.id, key.tid, key.id); > err++; > } > - > - cond_resched(); > } > return err; > } > @@ -660,8 +652,6 @@ static int threadfunc(void *data) > goto out; > } > tdata->objs[i].value.id = TEST_INSERT_FAIL; > - > - cond_resched(); > } > err = thread_lookup_test(tdata); > if (err) { > -- > 2.31.1 > -- Kees Cook
