Re: [LTP] [PATCH v2] mem/min_free_kbytes: Add grace period for memory reclaim

[Cyril Hrubis <chrubis@suse.cz>]

Hi!
> 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, particularly observed on
> openQA aarch64 machines.
> 
> Implement a 1-second grace period with exponential backoff polling
> (from 1ms up to 512ms) in check_monitor() to allow the kernel time to
> reclaim memory.
> 
> Reviewed-by: Petr Vorel <pvorel@suse.cz>
> Signed-off-by: Wei Gao <wegao@suse.com>
> ---
> v1->v2:
> - Combine TINFO and TFAIL messages in check_monitor() for cleaner output.
> - Remove end = 0;
> 
>  .../kernel/mem/tunable/min_free_kbytes.c      | 33 +++++++++++++------
>  1 file changed, 23 insertions(+), 10 deletions(-)
> 
> diff --git a/testcases/kernel/mem/tunable/min_free_kbytes.c b/testcases/kernel/mem/tunable/min_free_kbytes.c
> index a62e4ae9d..e0342ef06 100644
> --- a/testcases/kernel/mem/tunable/min_free_kbytes.c
> +++ b/testcases/kernel/mem/tunable/min_free_kbytes.c
> @@ -1,6 +1,6 @@
>  // SPDX-License-Identifier: GPL-2.0-or-later
>  /*
> - * Copyright (c) Linux Test Project, 2012-2025
> + * Copyright (c) Linux Test Project, 2012-2026
>   * Copyright (C) 2012-2017  Red Hat, Inc.
>   */
>  
> @@ -140,14 +140,13 @@ static void test_tune(unsigned long overcommit_policy)
>  		} else {
>  			if (WIFEXITED(status)) {
>  				if (WEXITSTATUS(status) != 0) {
> -					tst_res(TFAIL, "child unexpectedly "
> -						 "failed: %d", status);
> +					tst_res(TFAIL, "child unexpectedly failed: %d",
> +						status);

We do have tst_strstatus().

>  				}
>  			} else if (!WIFSIGNALED(status) ||
>  				   WTERMSIG(status) != SIGKILL) {
> -				tst_res(TFAIL,
> -					 "child unexpectedly failed: %d",
> -					 status);
> +				tst_res(TFAIL, "child unexpectedly failed: %d",
> +					status);
>  			}
>  		}
>  	}
> @@ -183,18 +182,32 @@ static void check_monitor(void)
>  {
>  	unsigned long tune;
>  	unsigned long memfree;
> +	int i;
>  
>  	while (!end) {
>  		memfree = SAFE_READ_MEMINFO("MemFree:");
>  		tune = TST_SYS_CONF_LONG_GET(MIN_FREE_KBYTES);
>  
>  		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");
> +			/*
> +			 * Give it some time to reclaim. The kernel should keep
> +			 * MemFree above min_free_kbytes, but transient drops
> +			 * are possible under high pressure.
> +			 */
> +			for (i = 1; i < 1024; i *= 2) {
> +				usleep(i * 1000);
> +				memfree = SAFE_READ_MEMINFO("MemFree:");
> +				if (memfree >= tune)
> +					break;
> +			}
> +
> +			if (memfree < tune) {
> +				tst_res(TFAIL, "MemFree %lu kB < min_free_kbytes %lu kB",
> +					memfree, tune);
> +			}
>  		}

Looks good.

Reviewed-by: Cyril Hrubis <chrubis@suse.cz>

I think that we also want to change the test so that the monitor is
started and stopped for each testcase with a specific value we set the
min_free_kbytes to. Running it asynchronously like this may mean that we
will be looking for a wrong value for the second if we are unlucky. But
that can be done later on.

-- 
Cyril Hrubis
chrubis@suse.cz

-- 
Mailing list info: https://lists.linux.it/listinfo/ltp