Fix malloc_utils features (sysmalloc) (#7770)

Follow-up to:

- https://github.com/sigp/lighthouse/pull/7764

The `heaptrack` feature added in my previous PR was ineffective, because the jemalloc feature was turned on by the Linux target-specific dependency.

This PR tweaks the features such that:

- The jemalloc feature is just used to control whether jemalloc is compiled in. It is enabled on Linux by the target-specific dependency (see `lighthouse/Cargo.toml`), and completely disabled on Windows.
- If the `sysmalloc` feature is set on Linux then it overrides jemalloc when selecting an allocator, _even if_ the jemalloc feature is enabled (and the jemalloc dep was compiled).

common/malloc_utils/src/lib.rs

@@ -25,28 +25,35 @@
 //! functions, then try to compile with the `not_glibc_interface` module.
 
 #[cfg(all(
+    any(feature = "sysmalloc", not(feature = "jemalloc")),
     target_os = "linux",
-    not(target_env = "musl"),
-    not(feature = "jemalloc")
+    not(target_env = "musl")
 ))]
 pub mod glibc;
 
-#[cfg(feature = "jemalloc")]
+#[cfg(all(unix, not(feature = "sysmalloc"), feature = "jemalloc"))]
 pub mod jemalloc;
 
 pub use interface::*;
 
+// Glibc malloc is the default on non-musl Linux if the sysmalloc feature is enabled, or jemalloc
+// is disabled.
 #[cfg(all(
+    any(feature = "sysmalloc", not(feature = "jemalloc")),
     target_os = "linux",
-    not(target_env = "musl"),
-    not(feature = "jemalloc")
+    not(target_env = "musl")
 ))]
 mod interface {
     pub use crate::glibc::configure_glibc_malloc as configure_memory_allocator;
     pub use crate::glibc::scrape_mallinfo_metrics as scrape_allocator_metrics;
+
+    pub fn allocator_name() -> String {
+        "glibc".to_string()
+    }
 }
 
-#[cfg(feature = "jemalloc")]
+// Jemalloc is the default on UNIX (including musl) unless the sysmalloc feature is enabled.
+#[cfg(all(unix, not(feature = "sysmalloc"), feature = "jemalloc"))]
 mod interface {
     #[allow(dead_code)]
     pub fn configure_memory_allocator() -> Result<(), String> {
@@ -54,11 +61,21 @@ mod interface {
     }
 
     pub use crate::jemalloc::scrape_jemalloc_metrics as scrape_allocator_metrics;
+
+    pub fn allocator_name() -> String {
+        match crate::jemalloc::page_size() {
+            Ok(page_size) => format!("jemalloc ({}K)", page_size / 1024),
+            Err(e) => format!("jemalloc (error: {e:?})"),
+        }
+    }
 }
 
-#[cfg(all(
-    any(not(target_os = "linux"), target_env = "musl"),
-    not(feature = "jemalloc")
+#[cfg(any(
+    not(unix),
+    all(
+        any(feature = "sysmalloc", not(feature = "jemalloc")),
+        any(not(target_os = "linux"), target_env = "musl")
+    )
 ))]
 mod interface {
     #[allow(dead_code, clippy::unnecessary_wraps)]
@@ -68,4 +85,8 @@ mod interface {
 
     #[allow(dead_code)]
     pub fn scrape_allocator_metrics() {}
+
+    pub fn allocator_name() -> String {
+        "system".to_string()
+    }
 }