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    SubjectRe: [RFC PATCH 71/86] treewide: lib: remove cond_resched()
    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@oracle.com/
    >
    > Cc: Herbert Xu <herbert@gondor.apana.org.au>
    > Cc: "David S. Miller" <davem@davemloft.net>
    > Cc: Kees Cook <keescook@chromium.org>
    > Cc: Eric Dumazet <edumazet@google.com>
    > Cc: Jakub Kicinski <kuba@kernel.org>
    > Cc: Paolo Abeni <pabeni@redhat.com>
    > Cc: Thomas Graf <tgraf@suug.ch>
    > Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
    > ---
    > 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

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