Merge pull request #20744 from yuwata/udev-netlink

udev: use netlink more aggressively

I'm pasting the comment from https://github.com/systemd/systemd/pull/20744#issuecomment-934485287
which is quite informative. The code wasn't changed significantly since then:

atenart commented 6 days ago:
> I ran tests without (93caec7) and with this PR (06735f2) on Fedora, having a few udev rules
> using attributes eligible to be cached and creating 50 veth on 4 CPUs. Although the time spent
> running the test is variable between runs, I generally saw an improvement when using this PR, e.g:
>
> 249-910-g93caec7:
> real	0m3.691s
> user	0m0.022s
> sys	0m1.338s
> 
> 249-920-g06735f2:
> real	0m2.950s
> user	0m0.005s
> sys	0m0.399s
> 
> On a different system than the one used above, I even saw a 40% improvement; results depend
> on many parameters (distro, udev rules, concurrent daemons accessing sysfs, etc.).
> 
> Because it's quite hard to measure the improvement here (as the kernel behave differently between
> the two test cases), I also ran tests using a modified kernel not hitting the trylock logic. There was
> an improvement with this PR as well. (Take this with a grain of salt though, as the kernel was
> modified not using patches approved upstream).

src/basic/ether-addr-util.c

@@ -7,6 +7,7 @@
 #include <sys/types.h>
 
 #include "ether-addr-util.h"
+#include "hexdecoct.h"
 #include "macro.h"
 #include "string-util.h"
 
@@ -15,12 +16,13 @@ char* hw_addr_to_string(const struct hw_addr_data *addr, char buffer[HW_ADDR_TO_
         assert(buffer);
         assert(addr->length <= HW_ADDR_MAX_SIZE);
 
-        for (size_t i = 0; i < addr->length; i++) {
-                sprintf(&buffer[3*i], "%02"PRIx8, addr->bytes[i]);
-                if (i < addr->length - 1)
-                        buffer[3*i + 2] = ':';
+        for (size_t i = 0, j = 0; i < addr->length; i++) {
+                buffer[j++] = hexchar(addr->bytes[i] >> 4);
+                buffer[j++] = hexchar(addr->bytes[i] & 0x0f);
+                buffer[j++] = ':';
         }
 
+        buffer[addr->length > 0 ? addr->length * 3 - 1 : 0] = '\0';
         return buffer;
 }