Re: [PATCH 2/6] selftests/resctrl: Ensure measurements skip initialization of default benchmark

On Thu, 29 Aug 2024, Reinette Chatre wrote:

The CMT, MBA, and MBM tests rely on the resctrl_val() wrapper to start and run a benchmark while providing test specific flows via callbacks to do test specific configuration and measurements.

At a high level, the resctrl_val() flow is: a) Start by fork()ing a child process that installs a signal handler for SIGUSR1 that, on receipt of SIGUSR1, will start running a benchmark. b) Assign the child process created in (a) to the resctrl control and monitoring group that dictates the memory and cache allocations with which the process can run and will contain all resctrl monitoring data of that process. c) Once parent and child are considered "ready" (determined via a message over a pipe) the parent signals the child (via SIGUSR1) to start the benchmark, waits one second for the benchmark to run, and then starts collecting monitoring data for the tests, potentially also changing allocation configuration depending on the various test callbacks.

A problem with the above flow is the "black box" view of the benchmark that is combined with an arbitrarily chosen "wait one second" before measurements start. No matter what the benchmark does, it is given one second to initialize before measurements start.

The default benchmark "fill_buf" consists of two parts, first it prepares a buffer (allocate, initialize, then flush), then it reads from the buffer (in unpredictable ways) until terminated. Depending on the system and the size of the buffer, the first "prepare" part may not be complete by the time the one second delay expires. Test measurements may thus start before the work needing to be measured runs.

Split the default benchmark into its "prepare" and "runtime" parts and simplify the resctrl_val() wrapper while doing so. This same split cannot be done for the user provided benchmark (without a user interface change), so the current behavior is maintained for user provided benchmark.

Assign the test itself to the control and monitoring group and run the "prepare" part of the benchmark in this context, ensuring it runs with required cache and memory bandwidth allocations. With the benchmark preparation complete it is only needed to fork() the "runtime" part of the benchmark (or entire user provided benchmark).

Keep the "wait one second" delay before measurements start. For the default "fill_buf" benchmark this time now covers only the "runtime" portion that needs to be measured. For the user provided benchmark this delay maintains current behavior.

Signed-off-by: Reinette Chatre reinette.chatre@intel.com

tools/testing/selftests/resctrl/fill_buf.c | 19 +- tools/testing/selftests/resctrl/resctrl.h | 2 +- tools/testing/selftests/resctrl/resctrl_val.c | 225 ++++++------------ 3 files changed, 70 insertions(+), 176 deletions(-)

diff --git a/tools/testing/selftests/resctrl/fill_buf.c b/tools/testing/selftests/resctrl/fill_buf.c index ae120f1735c0..12c71bb44cb6 100644 --- a/tools/testing/selftests/resctrl/fill_buf.c +++ b/tools/testing/selftests/resctrl/fill_buf.c @@ -114,7 +114,7 @@ void fill_cache_read(unsigned char *buf, size_t buf_size, bool once) *value_sink = ret; } -static void fill_cache_write(unsigned char *buf, size_t buf_size, bool once) +void fill_cache_write(unsigned char *buf, size_t buf_size, bool once) { while (1) { fill_one_span_write(buf, buf_size); @@ -150,20 +150,3 @@ unsigned char *alloc_buffer(size_t buf_size, int memflush) return buf; }

-int run_fill_buf(size_t buf_size, int memflush, int op, bool once) -{

• unsigned char *buf;
• buf = alloc_buffer(buf_size, memflush);
• if (!buf)

• return -1;
  

• if (op == 0)

• fill_cache_read(buf, buf_size, once);
  

• else

• fill_cache_write(buf, buf_size, once);
  

• free(buf);
• return 0;

-} diff --git a/tools/testing/selftests/resctrl/resctrl.h b/tools/testing/selftests/resctrl/resctrl.h index 2dda56084588..0afbc4dd18e4 100644 --- a/tools/testing/selftests/resctrl/resctrl.h +++ b/tools/testing/selftests/resctrl/resctrl.h @@ -142,7 +142,7 @@ int perf_event_open(struct perf_event_attr *hw_event, pid_t pid, int cpu, unsigned char *alloc_buffer(size_t buf_size, int memflush); void mem_flush(unsigned char *buf, size_t buf_size); void fill_cache_read(unsigned char *buf, size_t buf_size, bool once); -int run_fill_buf(size_t buf_size, int memflush, int op, bool once); +void fill_cache_write(unsigned char *buf, size_t buf_size, bool once); int initialize_mem_bw_imc(void); int measure_mem_bw(const struct user_params *uparams, struct resctrl_val_param *param, pid_t bm_pid, diff --git a/tools/testing/selftests/resctrl/resctrl_val.c b/tools/testing/selftests/resctrl/resctrl_val.c index 70e8e31f5d1a..574b72604f95 100644 --- a/tools/testing/selftests/resctrl/resctrl_val.c +++ b/tools/testing/selftests/resctrl/resctrl_val.c @@ -448,7 +448,7 @@ static int get_mem_bw_resctrl(FILE *fp, unsigned long *mbm_total) return 0; } -static pid_t bm_pid, ppid; +static pid_t bm_pid; void ctrlc_handler(int signum, siginfo_t *info, void *ptr) { @@ -506,13 +506,6 @@ void signal_handler_unregister(void) } } -static void parent_exit(pid_t ppid) -{

• kill(ppid, SIGKILL);
• umount_resctrlfs();
• exit(EXIT_FAILURE);

-}

/*

• print_results_bw: the memory bandwidth results are stored in a file
• @filename: file that stores the results

@@ -614,61 +607,6 @@ int measure_mem_bw(const struct user_params *uparams, return ret; } -/*

• • run_benchmark - Run a specified benchmark or fill_buf (default benchmark)

• •    in specified signal. Direct benchmark stdio to /dev/null.
    

• • @signum: signal number
• • @info: signal info
• • @ucontext: user context in signal handling
• */

-static void run_benchmark(int signum, siginfo_t *info, void *ucontext) -{

• int operation, ret, memflush;
• char **benchmark_cmd;
• size_t span;
• bool once;
• FILE *fp;
• benchmark_cmd = info->si_ptr;
• /*

• * Direct stdio of child to /dev/null, so that only parent writes to
  

• * stdio (console)
  

• */
  

• fp = freopen("/dev/null", "w", stdout);
• if (!fp) {

• ksft_perror("Unable to direct benchmark status to /dev/null");
  

• parent_exit(ppid);
  

• }
• if (strcmp(benchmark_cmd[0], "fill_buf") == 0) {

• /* Execute default fill_buf benchmark */
  

• span = strtoul(benchmark_cmd[1], NULL, 10);
  

• memflush =  atoi(benchmark_cmd[2]);
  

• operation = atoi(benchmark_cmd[3]);
  

• if (!strcmp(benchmark_cmd[4], "true")) {
  

• 	once = true;
  

• } else if (!strcmp(benchmark_cmd[4], "false")) {
  

• 	once = false;
  

• } else {
  

• 	ksft_print_msg("Invalid once parameter\n");
  

• 	parent_exit(ppid);
  

• }
  

• if (run_fill_buf(span, memflush, operation, once))
  

• 	fprintf(stderr, "Error in running fill buffer\n");
  

• } else {

• /* Execute specified benchmark */
  

• ret = execvp(benchmark_cmd[0], benchmark_cmd);
  

• if (ret)
  

• 	ksft_perror("execvp");
  

• }
• fclose(stdout);
• ksft_print_msg("Unable to run specified benchmark\n");
• parent_exit(ppid);

-}

/*

• resctrl_val: execute benchmark and measure memory bandwidth on

• 	the benchmark
  

@@ -684,11 +622,13 @@ int resctrl_val(const struct resctrl_test *test, const char * const *benchmark_cmd, struct resctrl_val_param *param) {

• struct sigaction sigact;
• int ret = 0, pipefd[2];
• char pipe_message = 0;
• union sigval value;
• int domain_id;

• int domain_id, operation = 0, memflush = 1;
• size_t span = DEFAULT_SPAN;
• unsigned char *buf = NULL;
• cpu_set_t old_affinity;
• bool once = false;
• int ret = 0;
• pid_t ppid;

if (strcmp(param->filename, "") == 0) sprintf(param->filename, "stdio"); @@ -699,111 +639,80 @@ int resctrl_val(const struct resctrl_test *test, return ret; }

• /*

• * If benchmark wasn't successfully started by child, then child should
  

• * kill parent, so save parent's pid
  

• */
  

  ppid = getpid();

• if (pipe(pipefd)) {

• ksft_perror("Unable to create pipe");
  

• /* Taskset test to specified CPU. */
• ret = taskset_benchmark(ppid, uparams->cpu, &old_affinity);

Previously only CPU affinity for bm_pid was set but now it's set before fork(). Quickly checking the Internet, it seems that CPU affinity gets inherited on fork() so now both processes will have the same affinity which might make the other process to interfere with the measurement.

• if (ret)

• return ret;
  

• return -1;
  

• /* Write test to specified control & monitoring group in resctrl FS. */
• ret = write_bm_pid_to_resctrl(ppid, param->ctrlgrp, param->mongrp);

Previously, this was done for bm_pid but now it's done for the parent. I'm not sure how inheritance goes with resctrl on fork(), will the forked PID get added to the list of PIDs or not? You probably know the answer :-). Neither behavior, however, seems to result in the intended behavior as we either get interfering processes (if inherited) or no desired resctrl setup for the benchmark process.

• if (ret)

• goto reset_affinity;
  

• if (param->init) {

• ret = param->init(param, domain_id);
  

• if (ret)
  

• 	goto reset_affinity;
  

  }

/*

• * Fork to start benchmark, save child's pid so that it can be killed
  

• * when needed
  

• * If not running user provided benchmark, run the default
  

• * "fill_buf". First phase of "fill_buf" is to prepare the
  

• * buffer that the benchmark will operate on. No measurements
  

• * are needed during this phase and prepared memory will be
  

• * passed to next part of benchmark via copy-on-write. TBD
  

• * how this impacts "write" benchmark, but no test currently
  

• * uses this.
  

  */

• fflush(stdout);

Please don't remove fflush() in front of fork() as it leads to duplicating messages.