lmkd: Optimize frequent file reads by keeping file descriptors open

To check system memory state lmkd is using same files every time vmpressure
event is received. Instead of opening and closing these files every time
we store the file descriptor and use pread() to reread the file from
the beginning.


Bug: 77299493
Bug: 75322373
Merged-In: I8e27f8b9526e82e3cc313a02fce215c2e4dd3d29
Change-Id: I8e27f8b9526e82e3cc313a02fce215c2e4dd3d29
Signed-off-by: Suren Baghdasaryan <surenb@google.com>

lmkd/lmkd.c

@@ -169,6 +169,11 @@ struct proc {
     struct proc *pidhash_next;
 };
 
+struct reread_data {
+    const char* const filename;
+    int fd;
+};
+
 #define PIDHASH_SZ 1024
 static struct proc *pidhash[PIDHASH_SZ];
 #define pid_hashfn(x) ((((x) >> 8) ^ (x)) & (PIDHASH_SZ - 1))
@@ -179,12 +184,27 @@ static struct adjslot_list procadjslot_list[ADJTOSLOT(OOM_SCORE_ADJ_MAX) + 1];
 /* PAGE_SIZE / 1024 */
 static long page_k;
 
+static bool parse_int64(const char* str, int64_t* ret) {
+    char* endptr;
+    long long val = strtoll(str, &endptr, 10);
+    if (str == endptr || val > INT64_MAX) {
+        return false;
+    }
+    *ret = (int64_t)val;
+    return true;
+}
+
+/*
+ * Read file content from the beginning up to max_len bytes or EOF
+ * whichever happens first.
+ */
 static ssize_t read_all(int fd, char *buf, size_t max_len)
 {
     ssize_t ret = 0;
+    off_t offset = 0;
 
     while (max_len > 0) {
-        ssize_t r = read(fd, buf, max_len);
+        ssize_t r = TEMP_FAILURE_RETRY(pread(fd, buf, max_len, offset));
         if (r == 0) {
             break;
         }
@@ -193,12 +213,44 @@ static ssize_t read_all(int fd, char *buf, size_t max_len)
         }
         ret += r;
         buf += r;
+        offset += r;
         max_len -= r;
     }
 
     return ret;
 }
 
+/*
+ * Read a new or already opened file from the beginning.
+ * If the file has not been opened yet data->fd should be set to -1.
+ * To be used with files which are read often and possibly during high
+ * memory pressure to minimize file opening which by itself requires kernel
+ * memory allocation and might result in a stall on memory stressed system.
+ */
+static int reread_file(struct reread_data *data, char *buf, size_t buf_size) {
+    ssize_t size;
+
+    if (data->fd == -1) {
+        data->fd = open(data->filename, O_RDONLY | O_CLOEXEC);
+        if (data->fd == -1) {
+            ALOGE("%s open: %s", data->filename, strerror(errno));
+            return -1;
+        }
+    }
+
+    size = read_all(data->fd, buf, buf_size - 1);
+    if (size < 0) {
+        ALOGE("%s read: %s", data->filename, strerror(errno));
+        close(data->fd);
+        data->fd = -1;
+        return -1;
+    }
+    ALOG_ASSERT((size_t)size < buf_size - 1, data->filename " too large");
+    buf[size] = 0;
+
+    return 0;
+}
+
 static struct proc *pid_lookup(int pid) {
     struct proc *procp;
 
@@ -460,7 +512,7 @@ static void ctrl_data_close(int dsock_idx) {
 static int ctrl_data_read(int dsock_idx, char *buf, size_t bufsz) {
     int ret = 0;
 
-    ret = read(data_sock[dsock_idx].sock, buf, bufsz);
+    ret = TEMP_FAILURE_RETRY(read(data_sock[dsock_idx].sock, buf, bufsz));
 
     if (ret == -1) {
         ALOGE("control data socket read failed; errno=%d", errno);
@@ -807,23 +859,19 @@ static int find_and_kill_processes(enum vmpressure_level level,
     return pages_freed;
 }
 
-static int64_t get_memory_usage(const char* path) {
+static int64_t get_memory_usage(struct reread_data *file_data) {
     int ret;
     int64_t mem_usage;
     char buf[32];
-    int fd = open(path, O_RDONLY | O_CLOEXEC);
-    if (fd == -1) {
-        ALOGE("%s open: errno=%d", path, errno);
+
+    if (reread_file(file_data, buf, sizeof(buf)) < 0) {
         return -1;
     }
 
-    ret = read_all(fd, buf, sizeof(buf) - 1);
-    close(fd);
-    if (ret < 0) {
-        ALOGE("%s error: errno=%d", path, errno);
+    if (!parse_int64(buf, &mem_usage)) {
+        ALOGE("%s parse error", file_data->filename);
         return -1;
     }
-    sscanf(buf, "%" SCNd64, &mem_usage);
     if (mem_usage == 0) {
         ALOGE("No memory!");
         return -1;
@@ -883,6 +931,14 @@ static void mp_event_common(int data, uint32_t events __unused) {
     static struct timeval last_report_tm;
     static unsigned long skip_count = 0;
     enum vmpressure_level level = (enum vmpressure_level)data;
+    static struct reread_data mem_usage_file_data = {
+        .filename = MEMCG_MEMORY_USAGE,
+        .fd = -1,
+    };
+    static struct reread_data memsw_usage_file_data = {
+        .filename = MEMCG_MEMORYSW_USAGE,
+        .fd = -1,
+    };
 
     /*
      * Check all event counters from low to critical
@@ -891,7 +947,8 @@ static void mp_event_common(int data, uint32_t events __unused) {
      */
     for (lvl = VMPRESS_LEVEL_LOW; lvl < VMPRESS_LEVEL_COUNT; lvl++) {
         if (mpevfd[lvl] != -1 &&
-            read(mpevfd[lvl], &evcount, sizeof(evcount)) > 0 &&
+            TEMP_FAILURE_RETRY(read(mpevfd[lvl],
+                               &evcount, sizeof(evcount))) > 0 &&
             evcount > 0 && lvl > level) {
             level = lvl;
         }
@@ -928,9 +985,10 @@ static void mp_event_common(int data, uint32_t events __unused) {
         return;
     }
 
-    mem_usage = get_memory_usage(MEMCG_MEMORY_USAGE);
-    memsw_usage = get_memory_usage(MEMCG_MEMORYSW_USAGE);
-    if (memsw_usage < 0 || mem_usage < 0) {
+    if ((mem_usage = get_memory_usage(&mem_usage_file_data)) < 0) {
+        goto do_kill;
+    }
+    if ((memsw_usage = get_memory_usage(&memsw_usage_file_data)) < 0) {
         goto do_kill;
     }