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Fix systemd-journal-gatewayd 100% CPU issue when watching logs (#4197)

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When follow request for logs is issued that points to/beyond the end of logs, a
busy loop in systemd-journal-gatewayd can be triggered which manifests as
systemd-journal-gatewayd consuming 100% CPU. Since threads are used for each
request, the logs may still work but the CPU will be hogged until the restart
of systemd-journal-gatewayd, Supervisor, or the whole system.

Backport the patch submitted upstream that addresses this issue.

Fixes #4190
This commit is contained in:
Jan Čermák 2025-07-31 11:06:59 +02:00 committed by GitHub
parent 10e401e2f6
commit 7e1e8b6f5d
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buildroot-external/patches/systemd/0007-journal-gatewayd-fix-busy-loop-when-following-way-be.patch Normal file
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@ -0,0 +1,88 @@
From d93da906a2148429f21c201aeb20e8738c22f4a4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jan=20=C4=8Cerm=C3=A1k?= <sairon@sairon.cz>
Date: Wed, 30 Jul 2025 19:18:13 +0200
Subject: [PATCH] journal-gatewayd: fix busy loop when following way beyond
journal end
Fix regression introduced in a7bfb9f76b96888d60b4f287f29dcbf758ba34c0,
where busy loop can be started with a request for following logs with a
range header set with num_skip value pointing beyond the end of the
journal. In that case the reader callback returns 0 and is called
immediately again, usually causing an endless loop that is not recovered
even when new journal events are added.
The bug does not occur if num_skip is not set - in that case if no
journal entries matching the filters are added, the tight loop is
avoided by the sd_journal_wait().
To fix the issue, when no matching journal events are available, set a
flag and reuse the backoff mechanism using the sd_journal_wait().
Link: https://github.com/home-assistant/operating-system/issues/4190
---
(Backported for v256.x)
Signed-off-by: Jan Čermák <sairon@sairon.cz>
Upstream: https://github.com/systemd/systemd/pull/38422
---
src/journal-remote/journal-gatewayd.c | 31 +++++++++++++++------------
1 file changed, 17 insertions(+), 14 deletions(-)
diff --git a/src/journal-remote/journal-gatewayd.c b/src/journal-remote/journal-gatewayd.c
index 5bce48d485..f7ecc352cc 100644
--- a/src/journal-remote/journal-gatewayd.c
+++ b/src/journal-remote/journal-gatewayd.c
@@ -171,6 +171,7 @@ static ssize_t request_reader_entries(
while (pos >= m->size) {
off_t sz;
+ bool wait_for_events = false;
/* End of this entry, so let's serialize the next
* one */
@@ -191,9 +192,10 @@ static ssize_t request_reader_entries(
* from it are not returned. */
if (r < m->n_skip + 1) {
m->n_skip -= r;
- if (m->follow)
- return 0;
- return MHD_CONTENT_READER_END_OF_STREAM;
+
+ if (!m->follow)
+ return MHD_CONTENT_READER_END_OF_STREAM;
+ wait_for_events = true;
}
} else
r = sd_journal_next(m->journal);
@@ -202,20 +204,21 @@ static ssize_t request_reader_entries(
log_error_errno(r, "Failed to advance journal pointer: %m");
return MHD_CONTENT_READER_END_WITH_ERROR;
} else if (r == 0) {
+ if (!m->follow)
+ return MHD_CONTENT_READER_END_OF_STREAM;
+ wait_for_events = true;
+ }
- if (m->follow) {
- r = sd_journal_wait(m->journal, (uint64_t) JOURNAL_WAIT_TIMEOUT);
- if (r < 0) {
- log_error_errno(r, "Couldn't wait for journal event: %m");
- return MHD_CONTENT_READER_END_WITH_ERROR;
- }
- if (r == SD_JOURNAL_NOP)
- break;
-
- continue;
+ if (wait_for_events) {
+ r = sd_journal_wait(m->journal, (uint64_t) JOURNAL_WAIT_TIMEOUT);
+ if (r < 0) {
+ log_error_errno(r, "Couldn't wait for journal event: %m");
+ return MHD_CONTENT_READER_END_WITH_ERROR;
}
+ if (r == SD_JOURNAL_NOP)
+ break;
- return MHD_CONTENT_READER_END_OF_STREAM;
+ continue;
}
if (m->discrete) {