diff --git a/include/host-build.mk b/include/host-build.mk
index 9673d5c03c9..b459c0ab18c 100644
--- a/include/host-build.mk
+++ b/include/host-build.mk
@@ -182,34 +182,42 @@ ifndef DUMP
   $(if $(DUMP),,$(call HostHost/Autoclean))
 
   $(HOST_STAMP_PREPARED):
+	$(call BuildTimeLog,begin,prepare)
 	@-rm -rf $(HOST_BUILD_DIR)
 	@mkdir -p $(HOST_BUILD_DIR)
 	$(foreach hook,$(Hooks/HostPrepare/Pre),$(call $(hook))$(sep))
 	$(call Host/Prepare)
 	$(foreach hook,$(Hooks/HostPrepare/Post),$(call $(hook))$(sep))
 	touch $$@
+	$(call BuildTimeLog,end,prepare)
 
   $(call Host/Exports,$(HOST_STAMP_CONFIGURED))
   $(HOST_STAMP_CONFIGURED): $(HOST_STAMP_PREPARED)
+	$(call BuildTimeLog,begin,configure)
 	$(foreach hook,$(Hooks/HostConfigure/Pre),$(call $(hook))$(sep))
 	$(call Host/Configure)
 	$(foreach hook,$(Hooks/HostConfigure/Post),$(call $(hook))$(sep))
 	touch $$@
+	$(call BuildTimeLog,end,configure)
 
   $(call Host/Exports,$(HOST_STAMP_BUILT))
   $(HOST_STAMP_BUILT): $(HOST_STAMP_CONFIGURED)
+		$(call BuildTimeLog,begin,compile)
 		$(foreach hook,$(Hooks/HostCompile/Pre),$(call $(hook))$(sep))
 		$(call Host/Compile)
 		$(foreach hook,$(Hooks/HostCompile/Post),$(call $(hook))$(sep))
 		touch $$@
+		$(call BuildTimeLog,end,compile)
 
   $(call Host/Exports,$(HOST_STAMP_INSTALLED))
   $(HOST_STAMP_INSTALLED): $(HOST_STAMP_BUILT) $(if $(FORCE_HOST_INSTALL),FORCE)
+		$(call BuildTimeLog,begin,install)
 		$(call Host/Install,$(HOST_BUILD_PREFIX))
 		$(foreach hook,$(Hooks/HostInstall/Post),$(call $(hook))$(sep))
 		mkdir -p $$(shell dirname $$@)
 		touch $(HOST_STAMP_BUILT)
 		touch $$@ $(HOST_STAMP_PROGRAMS)
+		$(call BuildTimeLog,end,install)
 
   $(call DefaultTargets,$(patsubst %,host-%,$(DEFAULT_SUBDIR_TARGETS)))
   ifndef STAMP_BUILT
diff --git a/include/kernel-build.mk b/include/kernel-build.mk
index 404c318d413..bd859185ee8 100644
--- a/include/kernel-build.mk
+++ b/include/kernel-build.mk
@@ -90,12 +90,15 @@ define BuildKernel
 
   $(Kernel/Autoclean)
   $(STAMP_PREPARED): $(if $(LINUX_SITE),$(DL_DIR)/$(LINUX_SOURCE))
+	$(call BuildTimeLog,begin,prepare)
 	-rm -rf $(KERNEL_BUILD_DIR)
 	-mkdir -p $(KERNEL_BUILD_DIR)
 	$(Kernel/Prepare)
 	touch $$@
+	$(call BuildTimeLog,end,prepare)
 
   $(KERNEL_BUILD_DIR)/symtab.h: FORCE
+	$(call BuildTimeLog,begin,compile)
 	rm -f $(KERNEL_BUILD_DIR)/symtab.h
 	touch $(KERNEL_BUILD_DIR)/symtab.h
 	+$(KERNEL_MAKE) vmlinux
@@ -124,26 +127,33 @@ define BuildKernel
 			awk '{print "*(___ksymtab_gpl+" $$$$1 ") \\" }'; \
 		echo; \
 	) > $$@
+	$(call BuildTimeLog,end,compile)
 
   $(STAMP_CONFIGURED): $(STAMP_PREPARED) $(LINUX_KCONFIG_LIST) $(TOPDIR)/.config FORCE
+	$(call BuildTimeLog,begin,configure)
 	$(Kernel/Configure)
 	touch $$@
+	$(call BuildTimeLog,end,configure)
 
   $(LINUX_DIR)/.modules: export STAGING_PREFIX=$$(STAGING_DIR_HOST)
   $(LINUX_DIR)/.modules: export PKG_CONFIG_PATH=$$(STAGING_DIR_HOST)/lib/pkgconfig
   $(LINUX_DIR)/.modules: export PKG_CONFIG_LIBDIR=$$(STAGING_DIR_HOST)/lib/pkgconfig
   $(LINUX_DIR)/.modules: export FAIL_ON_UNCONFIGURED=1
   $(LINUX_DIR)/.modules: $(STAMP_CONFIGURED) $(LINUX_DIR)/.config FORCE
+	$(call BuildTimeLog,begin,compile)
 	$(Kernel/CompileModules)
 	touch $$@
+	$(call BuildTimeLog,end,compile)
 
   $(LINUX_DIR)/.image: export STAGING_PREFIX=$$(STAGING_DIR_HOST)
   $(LINUX_DIR)/.image: export PKG_CONFIG_PATH=$$(STAGING_DIR_HOST)/lib/pkgconfig
   $(LINUX_DIR)/.image: export PKG_CONFIG_LIBDIR=$$(STAGING_DIR_HOST)/lib/pkgconfig
   $(LINUX_DIR)/.image: $(STAMP_CONFIGURED) $(if $(CONFIG_STRIP_KERNEL_EXPORTS),$(KERNEL_BUILD_DIR)/symtab.h) FORCE
+	$(call BuildTimeLog,begin,compile)
 	$(Kernel/CompileImage)
 	$(Kernel/CollectDebug)
 	touch $$@
+	$(call BuildTimeLog,end,compile)
 	
   mostlyclean: FORCE
 	$(Kernel/Clean)
diff --git a/include/package.mk b/include/package.mk
index 9f284741df6..fe213f80411 100644
--- a/include/package.mk
+++ b/include/package.mk
@@ -247,6 +247,7 @@ define Build/CoreTargets
 
   $(STAMP_PREPARED) : export PATH=$$(TARGET_PATH_PKG)
   $(STAMP_PREPARED): $(STAMP_PREPARED_DEPENDS)
+	$(call BuildTimeLog,begin,prepare)
 	@-rm -rf $(PKG_BUILD_DIR)
 	@mkdir -p $(PKG_BUILD_DIR)
 	touch $$@_check
@@ -254,18 +255,22 @@ define Build/CoreTargets
 	$(Build/Prepare)
 	$(foreach hook,$(Hooks/Prepare/Post),$(call $(hook))$(sep))
 	touch $$@
+	$(call BuildTimeLog,end,prepare)
 
   $(call Build/Exports,$(STAMP_CONFIGURED))
   $(STAMP_CONFIGURED): $(STAMP_PREPARED) $(STAMP_CONFIGURED_DEPENDS)
+	$(call BuildTimeLog,begin,configure)
 	rm -f $(STAMP_CONFIGURED_WILDCARD)
 	$(CleanStaging)
 	$(foreach hook,$(Hooks/Configure/Pre),$(call $(hook))$(sep))
 	$(Build/Configure)
 	$(foreach hook,$(Hooks/Configure/Post),$(call $(hook))$(sep))
 	touch $$@
+	$(call BuildTimeLog,end,configure)
 
   $(call Build/Exports,$(STAMP_BUILT))
   $(STAMP_BUILT): $(STAMP_CONFIGURED) $(STAMP_BUILT_DEPENDS)
+	$(call BuildTimeLog,begin,compile)
 	rm -f $$@
 	touch $$@_check
 	$(foreach hook,$(Hooks/Compile/Pre),$(call $(hook))$(sep))
@@ -274,9 +279,11 @@ define Build/CoreTargets
 	$(Build/Install)
 	$(foreach hook,$(Hooks/Install/Post),$(call $(hook))$(sep))
 	touch $$@
+	$(call BuildTimeLog,end,compile)
 
   $(STAMP_INSTALLED) : export PATH=$$(TARGET_PATH_PKG)
   $(STAMP_INSTALLED): $(STAMP_BUILT)
+	$(call BuildTimeLog,begin,install)
 	rm -rf $(STAGING_TMP_DIR)
 	mkdir -p $(STAGING_TMP_DIR)/host $(STAGING_DIR)/packages
 	$(foreach hook,$(Hooks/InstallDev/Pre),\
@@ -300,6 +307,7 @@ define Build/CoreTargets
 	fi
 	rm -rf $(STAGING_TMP_DIR)
 	touch $$@
+	$(call BuildTimeLog,end,install)
 
   ifdef Build/InstallDev
     $(_pkg_target)compile: $(STAMP_INSTALLED)
diff --git a/rules.mk b/rules.mk
index cef05638354..6a959c2b9d2 100644
--- a/rules.mk
+++ b/rules.mk
@@ -196,6 +196,16 @@ STAGING_DIR_IMAGE:=$(STAGING_DIR)/image
 BUILD_LOG_DIR:=$(if $(call qstrip,$(CONFIG_BUILD_LOG_DIR)),$(call qstrip,$(CONFIG_BUILD_LOG_DIR)),$(TOPDIR)/logs)
 PKG_INFO_DIR := $(STAGING_DIR)/pkginfo
 
+# Set BUILD_TIME_LOG=<file> in the environment to record a begin and an end
+# event for every prepare, configure, compile and install stage. Turn the log
+# into a report with scripts/build-time-report.pl.
+ifneq ($(BUILD_TIME_LOG),)
+  BUILD_TIME_LOG_FILE:=$(if $(filter /%,$(BUILD_TIME_LOG)),$(BUILD_TIME_LOG),$(TOPDIR)/$(BUILD_TIME_LOG))
+  BuildTimeLog = @$(SCRIPT_DIR)/build-time-log.sh $(BUILD_TIME_LOG_FILE) $(1) $(2) "$(if $(BUILD_SUBDIR),$(BUILD_SUBDIR),$(CURDIR))$(if $(BUILD_VARIANT),/$(BUILD_VARIANT))"
+else
+  BuildTimeLog =
+endif
+
 BUILD_DIR_HOST:=$(if $(IS_PACKAGE_BUILD),$(BUILD_DIR_BASE)/hostpkg,$(BUILD_DIR_BASE)/host)
 STAGING_DIR_HOST:=$(abspath $(STAGING_DIR)/../host)
 STAGING_DIR_HOSTPKG:=$(abspath $(STAGING_DIR)/../hostpkg)
diff --git a/scripts/build-time-log.sh b/scripts/build-time-log.sh
new file mode 100755
index 00000000000..20c61a261dd
--- /dev/null
+++ b/scripts/build-time-log.sh
@@ -0,0 +1,18 @@
+#!/bin/sh
+#
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Records one build stage event. See scripts/build-time-report.pl.
+#
+# Usage: build-time-log.sh <logfile> <begin|end> <stage> <name>
+
+[ $# -eq 4 ] || exit 0
+
+now=$(date +%s.%N 2>/dev/null)
+case "$now" in
+	''|*N*) now=$(date +%s) ;;
+esac
+
+printf '%s\t%s\t%s\t%s\n' "$now" "$2" "$3" "$4" >> "$1" 2>/dev/null
+
+exit 0
diff --git a/scripts/build-time-report.pl b/scripts/build-time-report.pl
new file mode 100755
index 00000000000..3f2e2a1c53b
--- /dev/null
+++ b/scripts/build-time-report.pl
@@ -0,0 +1,181 @@
+#!/usr/bin/env perl
+#
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Turns a BUILD_TIME_LOG file into a report.
+#
+# Usage: build-time-report.pl [-n <count>] [-s <stage>] <logfile>
+#
+# Wall share is the share of the build duration that a stage is responsible
+# for. Each second of the build is divided between the stages that run in it,
+# so the wall shares of all stages add up to the busy time. A stage that runs
+# alone on an idle machine gets the whole second. Solo time counts only the
+# seconds in which a stage is the one thing that runs, so it is the part of
+# the build that a faster stage shortens directly.
+
+use strict;
+use warnings;
+
+my $top = 20;
+my $only_stage;
+
+while (@ARGV && $ARGV[0] =~ /^-/) {
+	my $opt = shift @ARGV;
+	if ($opt eq '-n') { $top = shift @ARGV }
+	elsif ($opt eq '-s') { $only_stage = shift @ARGV }
+	else { die "Usage: $0 [-n <count>] [-s <stage>] <logfile>\n" }
+}
+
+my $file = shift @ARGV or die "Usage: $0 [-n <count>] [-s <stage>] <logfile>\n";
+
+open(my $fh, '<', $file) or die "$0: cannot read $file: $!\n";
+
+my (@iv, %pending);
+
+while (my $line = <$fh>) {
+	chomp $line;
+	my ($time, $event, $stage, $name) = split(/\t/, $line, 4);
+	next unless defined $name;
+	next unless defined $time && $time =~ /^[0-9]+(\.[0-9]+)?$/;
+	next if defined $only_stage && $stage ne $only_stage;
+
+	my $key = "$stage\t$name";
+
+	if ($event eq 'begin') {
+		push @{$pending{$key}}, $time;
+		next;
+	}
+
+	next unless $event eq 'end';
+	my $start = $pending{$key} ? pop @{$pending{$key}} : undef;
+	next unless defined $start;
+
+	push @iv, {
+		idx => scalar @iv,
+		start => $start,
+		end => $time,
+		dur => $time - $start,
+		stage => $stage,
+		name => $name,
+		share => 0,
+		solo => 0,
+	};
+}
+
+close($fh);
+
+my $incomplete = 0;
+$incomplete += scalar @{$pending{$_}} for keys %pending;
+
+die "$0: no completed stages in $file\n" unless @iv;
+
+my @events;
+for my $iv (@iv) {
+	push @events, [ $iv->{start}, 1, $iv ];
+	push @events, [ $iv->{end}, 0, $iv ];
+}
+
+# An end event sorts before a begin event at the same instant. One stage that
+# hands over to the next one must not look like parallel work.
+@events = sort { $a->[0] <=> $b->[0] || $a->[1] <=> $b->[1] } @events;
+
+my (%active, $idle, $busy);
+$idle = 0;
+$busy = 0;
+
+my $prev = $events[0][0];
+
+for my $ev (@events) {
+	my ($t, $is_start, $iv) = @$ev;
+	my $dt = $t - $prev;
+
+	if ($dt > 0) {
+		my @cur = values %active;
+		my $n = scalar @cur;
+
+		if (!$n) {
+			$idle += $dt;
+		}
+		else {
+			$busy += $dt;
+			my $part = $dt / $n;
+			$_->{share} += $part for @cur;
+			$cur[0]->{solo} += $dt if $n == 1;
+		}
+	}
+
+	if ($is_start) { $active{$iv->{idx}} = $iv }
+	else { delete $active{$iv->{idx}} }
+
+	$prev = $t;
+}
+
+my $total = $events[-1][0] - $events[0][0];
+my $sum_dur = 0;
+$sum_dur += $_->{dur} for @iv;
+
+my (%st, %pkg);
+
+for my $iv (@iv) {
+	my $s = $st{$iv->{stage}} ||= { count => 0, dur => 0, share => 0, solo => 0 };
+	$s->{count}++;
+	$s->{dur} += $iv->{dur};
+	$s->{share} += $iv->{share};
+	$s->{solo} += $iv->{solo};
+
+	my $key = sprintf("%s [%s]", $iv->{name}, $iv->{stage});
+	my $p = $pkg{$key} ||= { dur => 0, share => 0, solo => 0 };
+	$p->{dur} += $iv->{dur};
+	$p->{share} += $iv->{share};
+	$p->{solo} += $iv->{solo};
+}
+
+sub hms {
+	my $s = shift;
+	return sprintf("%d:%02d:%05.2f",
+		int($s / 3600), int($s / 60) % 60, $s - 60 * int($s / 60));
+}
+
+sub pct {
+	my ($a, $b) = @_;
+	return $b > 0 ? 100 * $a / $b : 0;
+}
+
+printf "build window      %s\n", hms($total);
+printf "busy              %s (%.1f%%)\n", hms($busy), pct($busy, $total);
+printf "idle              %s (%.1f%%)\n", hms($idle), pct($idle, $total);
+printf "stage time        %s\n", hms($sum_dur);
+printf "mean parallelism  %.2f\n", $busy > 0 ? $sum_dur / $busy : 0;
+printf "stages completed  %d\n", scalar @iv;
+printf "stages unfinished %d\n", $incomplete if $incomplete;
+print "\n";
+
+printf "%-10s %6s %12s %12s %8s %12s %8s\n",
+	'stage', 'count', 'sum', 'wall share', 'of wall', 'solo', 'of wall';
+
+for my $name (sort { $st{$b}{share} <=> $st{$a}{share} } keys %st) {
+	my $s = $st{$name};
+	printf "%-10s %6d %12s %12s %7.1f%% %12s %7.1f%%\n",
+		$name, $s->{count}, hms($s->{dur}), hms($s->{share}),
+		pct($s->{share}, $total), hms($s->{solo}), pct($s->{solo}, $total);
+}
+
+sub table {
+	my ($title, $key, $ref) = @_;
+
+	print "\n$title\n";
+	printf "%-56s %12s %12s %12s\n", 'stage', 'duration', 'wall share', 'solo';
+
+	my @k = sort { $ref->{$b}{$key} <=> $ref->{$a}{$key} } keys %$ref;
+	splice(@k, $top) if @k > $top;
+
+	for my $name (@k) {
+		my $e = $ref->{$name};
+		printf "%-56s %12s %12s %12s\n",
+			$name, hms($e->{dur}), hms($e->{share}), hms($e->{solo});
+	}
+}
+
+table("top $top by wall share", 'share', \%pkg);
+table("top $top by solo time", 'solo', \%pkg);
+table("top $top by duration", 'dur', \%pkg);