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tools/sched_ext: scx - Fix cmask_subset(), cmask_equal() and cmask_weight()

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cmask_equal(), cmask_weight() and cmask_subset() bounded their word walks
with CMASK_NR_WORDS(nr_cids), which pads by one word and can't tell the last
word in use without @base. The walks could thus cover a slack word past the
active range, which cmask_reframe() leaves non-zero: a stale bit there gave
cmask_equal() a spurious mismatch, cmask_weight() an inflated count, and
cmask_subset() a spurious violation. cmask_subset() could also read
@b->bits[] one word past its allocation (within the arena's fault-recovered
range, so harmless), and deviated from the kernel scx_cmask_subset() by
failing any @a range that doesn't nest inside @b's even when the overhanging
bits are all clear.

Bound the cmask_equal() and cmask_weight() walks by the words the range
actually spans, with early returns for empty ranges. Rewrite cmask_subset()
to match the kernel semantics: scan @a's overhangs for set bits with
cmask_next_set() and walk the words of the range intersection.
cmask_subset() moves below cmask_next_set(), which it now uses. Padding bits
don't need masking as every cmask helper keeps them clear.

Fixes: a58e6b79b4 ("sched_ext: Add cmask, a base-windowed bitmap over cid space")
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
This commit is contained in:
Tejun Heo 2026-07-09 11:08:41 -10:00
parent 49b3378a75
commit e6979d05c6
1 changed files with 55 additions and 33 deletions
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88
tools/sched_ext/include/scx/cid.bpf.h
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@ -391,7 +391,9 @@ static __always_inline bool cmask_equal(const struct scx_cmask __arena *a,
if (a->base != b->base || a->nr_cids != b->nr_cids)
return false;
nr_words = CMASK_NR_WORDS(a->nr_cids);
if (a->nr_cids == 0)
return true;
nr_words = (a->base + a->nr_cids - 1) / 64 - a->base / 64 + 1;
bpf_for(i, 0, CMASK_MAX_WORDS) {
if (i >= nr_words)
@ -402,36 +404,6 @@ static __always_inline bool cmask_equal(const struct scx_cmask __arena *a,
return true;
}
/*
* True iff every bit set in @a is also set in @b over the intersection of
* their ranges. Bits of @a outside @b's range fail the test.
*/
static __always_inline bool cmask_subset(const struct scx_cmask __arena *a,
const struct scx_cmask __arena *b)
{
u32 a_end = a->base + a->nr_cids;
u32 b_end = b->base + b->nr_cids;
u32 a_wbase = a->base / 64;
u32 b_wbase = b->base / 64;
u32 nr_words, i;
/* any bit of @a outside @b's range is a subset violation */
if (a->base < b->base || a_end > b_end)
return false;
nr_words = CMASK_NR_WORDS(a->nr_cids);
bpf_for(i, 0, CMASK_MAX_WORDS) {
u32 wi_b;
if (i >= nr_words)
break;
wi_b = a_wbase + i - b_wbase;
if (a->bits[i] & ~b->bits[wi_b])
return false;
}
return true;
}
/**
* cmask_next_set - find the first set bit at or after @cid
* @m: cmask to search
@ -488,16 +460,66 @@ static __always_inline u32 cmask_first_set(const struct scx_cmask __arena *m)
(cid) < (m)->base + (m)->nr_cids; \
(cid) = cmask_next_set((m), (cid) + 1))
/*
* True iff every bit set in @a is also set in @b. Matches the kernel-side
* scx_cmask_subset(): ranges don't need to nest, and set bits of @a outside
* @b's range fail the test.
*/
static __always_inline bool cmask_subset(const struct scx_cmask __arena *a,
const struct scx_cmask __arena *b)
{
u32 a_end = a->base + a->nr_cids;
u32 b_end = b->base + b->nr_cids;
u32 a_wbase = a->base / 64;
u32 b_wbase = b->base / 64;
u32 lo = a->base > b->base ? a->base : b->base;
u32 hi = a_end < b_end ? a_end : b_end;
u32 lo_word, hi_word, i;
/* set bits of @a outside @b's range can't be in @b */
if (a->base < b->base &&
cmask_next_set(a, a->base) < (b->base < a_end ? b->base : a_end))
return false;
if (a_end > b_end &&
cmask_next_set(a, a->base > b_end ? a->base : b_end) < a_end)
return false;
if (lo >= hi)
return true;
/*
* Walk the words the range intersection spans. Plain word tests
* suffice: the scans above guarantee @a has no set bit outside @b's
* range and padding bits are kept clear by all cmask helpers.
*/
lo_word = lo / 64;
hi_word = (hi - 1) / 64;
bpf_for(i, 0, CMASK_MAX_WORDS) {
u32 w = lo_word + i;
if (w > hi_word)
break;
if (a->bits[w - a_wbase] & ~b->bits[w - b_wbase])
return false;
}
return true;
}
/*
* Population count over [base, base + nr_cids). Padding bits in the head/tail
* words are guaranteed zero by the mutating helpers, so a flat popcount over
* all words is correct.
* the words the range spans is correct.
*/
static __always_inline u32 cmask_weight(const struct scx_cmask __arena *m)
{
u32 nr_words = CMASK_NR_WORDS(m->nr_cids), i;
u32 nr_words, i;
u32 count = 0;
if (!m->nr_cids)
return 0;
nr_words = (m->base + m->nr_cids - 1) / 64 - m->base / 64 + 1;
bpf_for(i, 0, CMASK_MAX_WORDS) {
if (i >= nr_words)
break;