[LTP] [PATCH 1/2] syscalls/msgstress03: fix fork failure on small memory systems

[xuyang2018.jy@fujitsu.com]

Hi Krzysztof
> Running syscalls/msgstress03 on a system with less than ~4 GB of RAM fails:
>
>      msgstress03    1  TFAIL  :  msgstress03.c:155: 	Fork failed (may be OK if under stress)
>
> In dmesg:
>
>      LTP: starting msgstress03
>      cgroup: fork rejected by pids controller in /user.slice/user-1000.slice/session-1.scope
>
> The reason is cgroups pid limit set by systemd user.slice.  The limit is
> set for login session, also for root user.  For example on 2 GB RAM
> machine it is set as:
>      /sys/fs/cgroup/pids/user.slice/user-0.slice/pids.max:5207
This file only exists on cgroupv1 and cgroupv2 should use
/sys/fs/cgroup/user.slice/user-0.slice/pids.max.

Also if some embedded system doesn't have systemd, do we have a generic 
way or fallback way to calculate the pid max?

For this problem, I have a calculate way in my patch[1] at this time 
last year.

[1]https://patchwork.ozlabs.org/project/ltp/patch/1592298082-21792-2-git-send-email-xuyang2018.jy@cn.fujitsu.com/

Best Regards
Yang Xu
>
> Read the maximum number of pids and adjust the test limit.  For 2 GB RAM
> machine with systemd this will result in:
>
>      msgstress03    0  TINFO  :  Found limit of processes 5056 (from /sys/fs/cgroup/pids/user.slice/user-1000.slice/pids.max)
>      msgstress03    0  TINFO  :  Requested number of processes higher than user session limit (10000>  4556), setting to 4556
>
> Signed-off-by: Krzysztof Kozlowski<krzysztof.kozlowski@canonical.com>
> ---
>   include/ipcmsg.h                              |  2 +
>   libs/libltpipc/libipc.c                       | 58 +++++++++++++++++++
>   .../syscalls/ipc/msgstress/msgstress03.c      | 15 ++++-
>   3 files changed, 74 insertions(+), 1 deletion(-)
>
> diff --git a/include/ipcmsg.h b/include/ipcmsg.h
> index d89894b726cf..b73b72d6d172 100644
> --- a/include/ipcmsg.h
> +++ b/include/ipcmsg.h
> @@ -61,8 +61,10 @@ void rm_queue(int);
>
>   key_t getipckey();
>   int getuserid(char *);
> +int get_session_uid(void);
>
>   int get_max_msgqueues(void);
>   int get_used_msgqueues(void);
> +int get_pids_limit(void);
>
>   #endif /* ipcmsg.h */
> diff --git a/libs/libltpipc/libipc.c b/libs/libltpipc/libipc.c
> index d94880f54b64..cd3480ed9f21 100644
> --- a/libs/libltpipc/libipc.c
> +++ b/libs/libltpipc/libipc.c
> @@ -151,6 +151,31 @@ int getuserid(char *user)
>   	return (ent->pw_uid);
>   }
>
> +/*
> + * Get the effective session UID - either one invoking current test via sudo
> + * or the real UID.
> + */
> +int get_session_uid(void)
> +{
> +	const char *sudo_uid;
> +
> +	sudo_uid = getenv("SUDO_UID");
> +	if (sudo_uid) {
> +		int real_uid;
> +
> +		TEST(sscanf(sudo_uid, "%u",&real_uid));
> +		if (TEST_RETURN != 1) {
> +#ifdef DEBUG
> +			tst_resm(TINFO, "No SUDO_UID from env");
> +#endif
> +		} else {
> +			return real_uid;
> +		}
> +	}
> +
> +	return getuid();
> +}
> +
>   /*
>    * rm_shm() - removes a shared memory segment.
>    */
> @@ -218,3 +243,36 @@ int get_max_msgqueues(void)
>   	fclose(f);
>   	return atoi(buff);
>   }
> +
> +/*
> + * Get the limit of processes for current session configured by systemd user.slice.
> + * This also applies to root user.
> + */
> +int get_pids_limit(void)
> +{
> +	int real_uid, ret;
> +	char path[PATH_MAX];
> +	long unsigned int max_pids;
> +
> +	real_uid = get_session_uid();
> +	if (TEST_RETURN != 1) {
> +		tst_resm(TINFO, "Cannot get UID");
> +		return -1;
> +	}
> +
> +	ret = snprintf(path, sizeof(path),
> +		       "/sys/fs/cgroup/pids/user.slice/user-%d.slice/pids.max",
> +		       real_uid);
> +	if (ret<  0 || (size_t)ret>= sizeof(path))
> +		return -1;
> +
> +	if (access(path, R_OK) != 0) {
> +		tst_resm(TINFO, "Cannot read session user limits from '%s'", path);
> +		return -1;
> +	}
> +
> +	SAFE_FILE_SCANF(cleanup, path, "%lu",&max_pids);
> +	tst_resm(TINFO, "Found limit of processes %lu (from %s)", max_pids, path);
> +
> +	return max_pids;
> +}
> diff --git a/testcases/kernel/syscalls/ipc/msgstress/msgstress03.c b/testcases/kernel/syscalls/ipc/msgstress/msgstress03.c
> index 294b401b1b38..9cf96db7956e 100644
> --- a/testcases/kernel/syscalls/ipc/msgstress/msgstress03.c
> +++ b/testcases/kernel/syscalls/ipc/msgstress/msgstress03.c
> @@ -78,7 +78,7 @@ static void usage(void)
>
>   int main(int argc, char **argv)
>   {
> -	int i, j, ok, pid;
> +	int i, j, ok, pid, max_session_pids;
>   	int count, status;
>   	struct sigaction act;
>
> @@ -109,6 +109,19 @@ int main(int argc, char **argv)
>   		}
>   	}
>
> +	max_session_pids = get_pids_limit();
> +	if (max_session_pids>  0) {
> +		/* Clamp number of processes to session limit with some buffer for OS */
> +		max_session_pids = (max_session_pids>  500 ? max_session_pids - 500 : 0);
> +		if (nprocs>= max_session_pids) {
> +			tst_resm(TINFO,
> +				 "Requested number of processes higher than user session limit (%d>  %d), "
> +				 "setting to %d", nprocs, max_session_pids,
> +				 max_session_pids);
> +			nprocs = max_session_pids;
> +		}
> +	}
> +
>   	srand(getpid());
>   	tid = -1;
>