Re: [PATCH v5] min_free_kbytes: Handle transient memory drops in check_monitor

Hi Wei,

> High memory pressure can cause MemFree to temporarily drop below the
> min_free_kbytes threshold before the kernel reclaimer can catch up.
> This results in intermittent test failures, observed on openQA aarch64
> virtual machines.

> Implement a 2-second grace period with high-accuracy 10ms fixed polling
> in check_monitor() to allow the kernel time to reclaim memory.

> Introduce a 10% tolerance (90% threshold) for the MemFree check.
> Measurements under extreme pressure show MemFree can dip as low as
> ~50% to ~70% of the target. While it typically recovers above 90%
> within one second, hitting the exact 100% watermark sometimes can take
> significantly longer. This tolerance prevents false positives during the
> slow recovery tail while still ensuring memory is maintained near the
> required level.

> Also, increase the monitor's idle polling frequency from 2s to 100ms
> to improve responsiveness during the test run.

+1

> Enhanced diagnostics are added to report the minimum memory level seen
> during the pressure period to aid in future calibration.

> +++ b/testcases/kernel/mem/tunable/min_free_kbytes.c
...
>  	while (!end) {
>  		memfree = SAFE_READ_MEMINFO("MemFree:");
>  		tune = TST_SYS_CONF_LONG_GET(PATH_VM_MIN_FREE_KBYTES);
> +		/*
> +		 * Allow 10% tolerance to account for transient states.
> +		 */
> +		threshold = tune * 9 / 10;

>  		if (memfree < tune) {
> -			tst_res(TINFO, "MemFree is %lu kB, "
> -				"min_free_kbytes is %lu kB", memfree, tune);
> -			tst_res(TFAIL, "MemFree < min_free_kbytes");
> -			violated = 1;
> +			min_memfree = memfree;
> +			/*
> +			 * Give it some time to reclaim. The kernel should keep
> +			 * MemFree above min_free_kbytes, but transient drops
> +			 * are possible under high pressure.
> +			 * Check every 10ms for up to 2 seconds for high accuracy.
> +			 */
> +			for (i = 10; i <= 2000; i += 10) {
> +				usleep(10000);
You used i for usleep() in v1 and v2 [1]. Since v3 [2] you keep using
usleep(10000), therefore we could have just simple loop (readability):

			for (i = 0; i < 200; i++) {

Otherwise LGTM, thanks! (I don't agree with AI that 90% threshold is
hiding bugs). Please repost, so that we have patch in the new patchwork instance
in patchwork.kernel.org (+ please create an user on that instance if you haven't
already done), because old patchwork don't receive emails any more.

Reviewed-by: Petr Vorel <[email protected]>

Kind regards,
Petr

[1] https://lore.kernel.org/ltp/20260527050843.14963-1-wegao-IBi9RG/[email protected]/
[2] https://lore.kernel.org/ltp/20260531134250.25279-1-wegao-IBi9RG/[email protected]/

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