offload: Improve load normalization and configurability.

Fixes <https://issues.guix.gnu.org/43773>.

The computed normalized load was previously obtained by dividing the load
average as found in /proc/loadavg by the number of parallel builds defined for
a build machine.

This normalized load didn't allow to compare machines with different number of
cores, as the load average reported by /proc/loadavg can be as high as the
number of cores; thus comparing that value to a fixed threshold of 2.0 would
mean machines with multiple cores were more likely to be flagged as overloaded
compared to single core machines.

This can be fixed by normalizing using the available number of cores instead
of the number of parallel jobs.

* guix/scripts/offload.scm (<build-machine>)[overload-threshold]: New field.
(node-load): Modify to return a normalized load value between 0 and 1, taking
into account the number of cores available.
(normalized-load): Remove procedure.
(report-load): New procedure.
(choose-build-machine): Adjust to use the modified 'node-load' and the new
'report-load' and 'build-machine-overload-threshold' procedures.
(check-machine-status): Adjust.
* doc/guix.texi (Daemon Offload Setup): Document the offload scheduler and the
new 'overload-threshold' field.

doc/guix.texi

@@ -1081,7 +1081,28 @@ architecture natively supports it, via emulation (@pxref{Transparent
 Emulation with QEMU}), or both.  Missing prerequisites for the build are
 copied over SSH to the target machine, which then proceeds with the
 build; upon success the output(s) of the build are copied back to the
-initial machine.
+initial machine.  The offload facility comes with a basic scheduler that
+attempts to select the best machine.  The best machine is chosen among
+the available machines based on criteria such as:
+
+@enumerate
+@item
+The availability of a build slot.  A build machine can have as many
+build slots (connections) as the value of the @code{parallel-builds}
+field of its @code{build-machine} object.
+
+@item
+Its relative speed, as defined via the @code{speed} field of its
+@code{build-machine} object.
+
+@item
+Its load.  The normalized machine load must be lower than a threshold
+value, configurable via the @code{overload-threshold} field of its
+@code{build-machine} object.
+
+@item
+Disk space availability.  More than a 100 MiB must be available.
+@end enumerate
 
 The @file{/etc/guix/machines.scm} file typically looks like this:
 
@@ -1185,6 +1206,13 @@ when transferring files to and from build machines.
 File name of the Unix-domain socket @command{guix-daemon} is listening
 to on that machine.
 
+@item @code{overload-threshold} (default: @code{0.6})
+The load threshold above which a potential offload machine is
+disregarded by the offload scheduler.  The value roughly translates to
+the total processor usage of the build machine, ranging from 0.0 (0%) to
+1.0 (100%).  It can also be disabled by setting
+@code{overload-threshold} to @code{#f}.
+
 @item @code{parallel-builds} (default: @code{1})
 The number of builds that may run in parallel on the machine.
 

guix/scripts/offload.scm

@@ -88,6 +88,10 @@
                      (default 3))
   (daemon-socket   build-machine-daemon-socket    ; string
                    (default "/var/guix/daemon-socket/socket"))
+  ;; A #f value tells the offload scheduler to disregard the load of the build
+  ;; machine when selecting the best offload machine.
+  (overload-threshold build-machine-overload-threshold ; inexact real between
+                      (default 0.6))                   ; 0.0 and 1.0 | #f
   (parallel-builds build-machine-parallel-builds  ; number
                    (default 1))
   (speed           build-machine-speed            ; inexact real
@@ -391,30 +395,34 @@ of free disk space on '~a'~%")
   (* 100 (expt 2 20)))                            ;100 MiB
 
 (define (node-load node)
-  "Return the load on NODE.  Return +∞ if NODE is misbehaving."
+  "Return the load on NODE, a normalized value between 0.0 and 1.0.  The value
+is derived from /proc/loadavg and normalized according to the number of
+logical cores available, to give a rough estimation of CPU usage.  Return
+1.0 (fully loaded) if NODE is misbehaving."
   (let ((line (inferior-eval '(begin
                                 (use-modules (ice-9 rdelim))
                                 (call-with-input-file "/proc/loadavg"
                                   read-string))
-                             node)))
-    (if (eof-object? line)
-        +inf.0 ;MACHINE does not respond, so assume it is infinitely loaded
+                             node))
+        (ncores (inferior-eval '(begin
+                                  (use-modules (ice-9 threads))
+                                  (current-processor-count))
+                               node)))
+    (if (or (eof-object? line) (eof-object? ncores))
+        1.0    ;MACHINE does not respond, so assume it is fully loaded
         (match (string-tokenize line)
           ((one five fifteen . x)
-           (string->number one))
+           (let ((load (/ (string->number one) ncores)))
+             (if (> load 1.0)
+                 1.0
+                 load)))
           (x
-           +inf.0)))))
+           1.0)))))
 
-(define (normalized-load machine load)
-  "Divide LOAD by the number of parallel builds of MACHINE."
-  (if (rational? load)
-      (let* ((jobs       (build-machine-parallel-builds machine))
-             (normalized (/ load jobs)))
-        (format (current-error-port) "load on machine '~a' is ~s\
- (normalized: ~s)~%"
-                (build-machine-name machine) load normalized)
-        normalized)
-      load))
+(define (report-load machine load)
+  (format (current-error-port)
+          "normalized load on machine '~a' is ~,2f~%"
+          (build-machine-name machine) load))
 
 (define (random-seed)
   (logxor (getpid) (car (gettimeofday))))
@@ -472,11 +480,15 @@ slot (which must later be released with 'release-build-slot'), or #f and #f."
        (let* ((session (false-if-exception (open-ssh-session best
                                                              %short-timeout)))
               (node    (and session (remote-inferior session)))
-              (load    (and node (normalized-load best (node-load node))))
+              (load    (and node (node-load node)))
+              (threshold (build-machine-overload-threshold best))
               (space   (and node (node-free-disk-space node))))
+         (when load (report-load best load))
          (when node (close-inferior node))
          (when session (disconnect! session))
-         (if (and node (< load 2.) (>= space %minimum-disk-space))
+         (if (and node
+                  (or (not threshold) (< load threshold))
+                  (>= space %minimum-disk-space))
              (match others
                (((machines slots) ...)
                 ;; Release slots from the uninteresting machines.
@@ -708,13 +720,13 @@ machine."
                               (free (node-free-disk-space inferior)))
                           (close-inferior inferior)
                           (format #t "~a~%  kernel: ~a ~a~%  architecture: ~a~%\
-  host name: ~a~%  normalized load: ~a~%  free disk space: ~,2f MiB~%\
+  host name: ~a~%  normalized load: ~,2f~%  free disk space: ~,2f MiB~%\
   time difference: ~a s~%"
                                   (build-machine-name machine)
                                   (utsname:sysname uts) (utsname:release uts)
                                   (utsname:machine uts)
                                   (utsname:nodename uts)
-                                  (normalized-load machine load)
+                                  load
                                   (/ free (expt 2 20) 1.)
                                   (- time now))))))))