From: "Guilherme G. Piccoli" gpiccoli@igalia.com
[ Upstream commit 08d7becc1a6b8c936e25d827becabfe3bff72a36 ]
Right now, if the clocksource watchdog detects a clocksource skew, it might perform a per CPU check, for example in the TSC case on x86. In other words: supposing TSC is detected as unstable by the clocksource watchdog running at CPU1, as part of marking TSC unstable the kernel will also run a check of TSC readings on some CPUs to be sure it is synced between them all.
But that check happens only on some CPUs, not all of them; this choice is based on the parameter "verify_n_cpus" and in some random cpumask calculation. So, the watchdog runs such per CPU checks on up to "verify_n_cpus" random CPUs among all online CPUs, with the risk of repeating CPUs (that aren't double checked) in the cpumask random calculation.
But if "verify_n_cpus" > num_online_cpus(), it should skip the random calculation and just go ahead and check the clocksource sync between all online CPUs, without the risk of skipping some CPUs due to duplicity in the random cpumask calculation.
Tests in a 4 CPU laptop with TSC skew detected led to some cases of the per CPU verification skipping some CPU even with verify_n_cpus=8, due to the duplicity on random cpumask generation. Skipping the randomization when the number of online CPUs is smaller than verify_n_cpus, solves that.
Suggested-by: Thadeu Lima de Souza Cascardo cascardo@igalia.com Signed-off-by: Guilherme G. Piccoli gpiccoli@igalia.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Reviewed-by: Paul E. McKenney paulmck@kernel.org Link: https://lore.kernel.org/all/20250323173857.372390-1-gpiccoli@igalia.com Signed-off-by: Sasha Levin sashal@kernel.org ---
**YES** This commit should be backported to stable kernel trees. Here's my detailed analysis: **Bug Fix Analysis:** The commit fixes a logical flaw in the clocksource watchdog's per-CPU verification mechanism. The issue occurs in the `clocksource_verify_choose_cpus()` function at `/kernel/time/clocksource.c:313`. **The Problem:** The original condition `if (n < 0)` only checked for negative values of `verify_n_cpus`, but when `verify_n_cpus` (default 8) is greater than the number of online CPUs, the code would still proceed with random CPU selection. This could lead to: 1. **CPU duplication**: Random selection could pick the same CPU multiple times 2. **Incomplete coverage**: Some CPUs might never be tested due to duplicates 3. **Inefficient verification**: Testing fewer unique CPUs than intended **The Fix:** The change from: ```c if (n < 0) { ``` to: ```c if (n < 0 || n >= num_online_cpus()) { ``` Now correctly handles the case where `verify_n_cpus >= num_online_cpus()` by bypassing random selection and testing ALL online CPUs instead. **Backport Suitability Criteria:** 1. **Important Bug Fix**: ✅ Fixes incorrect CPU verification that could miss clocksource synchronization issues 2. **Small and Contained**: ✅ Single line change with clear, minimal scope 3. **No Side Effects**: ✅ Only affects the CPU selection logic, doesn't change fundamental behavior 4. **No Architectural Changes**: ✅ Simple conditional logic fix 5. **Critical Subsystem**: ✅ Clocksource watchdog is crucial for system timing reliability 6. **Minimal Regression Risk**: ✅ The fix makes the verification more thorough, not less **Similar Commit Patterns:** This matches the pattern of similar commits marked "YES" like commit #2 and #3, which were small, focused fixes to clocksource verification logic that improved reliability without introducing risks. **Real-World Impact:** The commit message specifically mentions testing on a 4-CPU laptop where the bug caused CPU verification to skip CPUs even with `verify_n_cpus=8`, demonstrating this is a real issue affecting actual systems. This is exactly the type of targeted bug fix that stable trees are designed to include - it fixes incorrect behavior, has minimal risk, and improves system reliability.
kernel/time/clocksource.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index b22508c5d2d96..bd49fec0f624b 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -273,7 +273,7 @@ static void clocksource_verify_choose_cpus(void) { int cpu, i, n = verify_n_cpus;
- if (n < 0) { + if (n < 0 || n >= num_online_cpus()) { /* Check all of the CPUs. */ cpumask_copy(&cpus_chosen, cpu_online_mask); cpumask_clear_cpu(smp_processor_id(), &cpus_chosen);