From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id C4C86D29DFE for ; Tue, 13 Jan 2026 10:19:53 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1vfbUp-0000UU-Ud; Tue, 13 Jan 2026 05:19:32 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vfbUk-0000ND-51 for qemu-rust@nongnu.org; Tue, 13 Jan 2026 05:19:27 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1vfbUi-00010K-Er for qemu-rust@nongnu.org; Tue, 13 Jan 2026 05:19:25 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1768299563; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=D7xhkILgIoQhxPiu+UT3981CPmnY2WGzO8z4JIZhcik=; b=F+4cAvib8CHisEwtHNRix/PykRNSNDenHjdlt+JLI3wummqA9zISNMwuCfQcXyy2RzRsy8 cJ/8YySaCWHPf02LHglXHz9BhZxagoolj80Z0zzuCbVAwesc62h1r203Np7QrUjiHUcQOG 9s7vBwhApo4kdsBuGcS/x4Nv1+zfOnA= Received: from mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-306-mq8SLJJxNqiz_zmfzMzKKQ-1; Tue, 13 Jan 2026 05:19:18 -0500 X-MC-Unique: mq8SLJJxNqiz_zmfzMzKKQ-1 X-Mimecast-MFC-AGG-ID: mq8SLJJxNqiz_zmfzMzKKQ_1768299557 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id B5BE419560AA; Tue, 13 Jan 2026 10:19:16 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.45.242.32]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id B02C719560B2; Tue, 13 Jan 2026 10:19:15 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 4941221E676C; Tue, 13 Jan 2026 11:19:13 +0100 (CET) From: Markus Armbruster To: Daniel P. =?utf-8?Q?Berrang=C3=A9?= Cc: qemu-devel@nongnu.org, Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , Manos Pitsidianakis , Hanna Reitz , Gerd Hoffmann , Paolo Bonzini , Christian Schoenebeck , "Dr. David Alan Gilbert" , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , devel@lists.libvirt.org, qemu-block@nongnu.org, qemu-rust@nongnu.org, Stefan Weil , Kevin Wolf , Richard Henderson Subject: Re: [PATCH v5 10/24] util: avoid repeated prefix on incremental qemu_log calls In-Reply-To: <20260108170338.2693853-11-berrange@redhat.com> ("Daniel P. =?utf-8?Q?Berrang=C3=A9=22's?= message of "Thu, 8 Jan 2026 17:03:24 +0000") References: <20260108170338.2693853-1-berrange@redhat.com> <20260108170338.2693853-11-berrange@redhat.com> Date: Tue, 13 Jan 2026 11:19:13 +0100 Message-ID: <87h5spak7y.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass client-ip=170.10.133.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-rust@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: QEMU Rust-related patches and discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-rust-bounces+qemu-rust=archiver.kernel.org@nongnu.org Sender: qemu-rust-bounces+qemu-rust=archiver.kernel.org@nongnu.org Daniel P. Berrang=C3=A9 writes: > There are three general patterns to QEMU log output > > 1. Single complete message calls > > qemu_log("Some message\n"); > > 2. Direct use of fprintf > > FILE *f =3D qemu_log_trylock() > fprintf(f, "..."); > fprintf(f, "..."); > fprintf(f, "...\n"); > qemu_log_unlock(f) > > 3. Mixed use of qemu_log_trylock/qemu_log() > > FILE *f =3D qemu_log_trylock() > qemu_log("...."); > qemu_log("...."); > qemu_log("....\n"); > qemu_log_unlock(f) > > When message prefixes are enabled, the timestamp will be > unconditionally emitted for all qemu_log() calls. This > works fine in the 1st case, and has no effect in the 2nd > case. In the 3rd case, however, we get the timestamp > printed over & over in each fragment. > > One can suggest that pattern (3) is pointless as it is > functionally identical to (2) but with extra indirection > and overhead. None the less we have a fair bit of code > that does this. > > The qemu_log() call itself is nothing more than a wrapper > which does pattern (2) with a single fprintf() call. > > One might question whether (2) should include the message > prefix in the same way that (1), but there are scenarios > where this could be inappropriate / unhelpful such as the > CPU register dumps or linux-user strace output. > > This patch fixes the problem in pattern (3) by keeping > track of the call depth of qemu_log_trylock() and then > only emitting the the prefix when the starting depth > was zero. In doing this qemu_log_trylock_context() is > also introduced as a variant of qemu_log_trylock() > that emits the prefix. Callers doing to batch output > can thus choose whether a prefix is appropriate or > not. I dislike pattern (3), but not nearly enough to chase it out of the tree myself. Hard to disagree with this patch then :) > Fixes: 012842c07552 (log: make '-msg timestamp=3Don' apply to all qemu_lo= g usage) > Reported-by: Richard Henderson > Signed-off-by: Daniel P. Berrang=C3=A9 > --- > include/qemu/log.h | 7 +++++++ > util/log.c | 49 ++++++++++++++++++++++++++-------------------- > 2 files changed, 35 insertions(+), 21 deletions(-) > > diff --git a/include/qemu/log.h b/include/qemu/log.h > index e9d3c6806b..95f417c2b7 100644 > --- a/include/qemu/log.h > +++ b/include/qemu/log.h > @@ -69,6 +69,13 @@ bool qemu_log_separate(void); > */ > FILE *qemu_log_trylock(void) G_GNUC_WARN_UNUSED_RESULT; >=20=20 > +/** > + * As qemu_log_trylock(), but will also print the message > + * context, if any is configured and this caused the > + * acquisition of the FILE lock > + */ > +FILE *qemu_log_trylock_context(void) G_GNUC_WARN_UNUSED_RESULT; This is used just once, in qemu_log(). Could be inlined into qemu_log(). If you prefer to keep it as a function, you could make it static. I wouldn't guess that this prints from the function name. Dunno, qemu_log_start_line()? Might want to pair it with a qemu_log_end_line() then. Telling function name is less important if it's static. > + > /** > * Releases the lock on the log output, previously > * acquired by qemu_log_trylock(). > diff --git a/util/log.c b/util/log.c > index c44d66b5ce..2ce7286f31 100644 > --- a/util/log.c > +++ b/util/log.c > @@ -127,13 +127,39 @@ static FILE *qemu_log_trylock_with_err(Error **errp) Not this patch's fault, but here goes anyway: static FILE *qemu_log_trylock_with_err(Error **errp) { FILE *logfile; logfile =3D thread_file; if (!logfile) { if (log_per_thread) { g_autofree char *filename =3D g_strdup_printf(global_filename, log_thread_id()); logfile =3D fopen(filename, "w"); if (!logfile) { error_setg_errno(errp, errno, "Error opening logfile %s for thread %d= ", filename, log_thread_id()); return NULL; } thread_file =3D logfile; qemu_log_thread_cleanup_notifier.notify =3D qemu_log_thread_= cleanup; qemu_thread_atexit_add(&qemu_log_thread_cleanup_notifier); } else { rcu_read_lock(); /* * FIXME: typeof_strip_qual, as used by qatomic_rcu_read, * does not work with pointers to undefined structures, * such as we have with struct _IO_FILE and musl libc. * Since all we want is a read of a pointer, cast to void**, * which does work with typeof_strip_qual. */ logfile =3D qatomic_rcu_read((void **)&global_file); if (!logfile) { rcu_read_unlock(); return NULL; Fails without setting @errp. } } } qemu_flockfile(logfile); > return logfile; > } >=20=20 > +/* > + * Zero if there's been no opening qemu_log_trylock call, > + * indicating the need for message context to be emitted > + * > + * Non-zero if we're in the middle of printing a message, > + * possibly over multiple lines and must skip further > + * message context > + */ > +static __thread uint log_depth; > + > FILE *qemu_log_trylock(void) > { > - return qemu_log_trylock_with_err(NULL); > + FILE *f =3D qemu_log_trylock_with_err(NULL); > + log_depth++; > + return f; > +} > + > +FILE *qemu_log_trylock_context(void) > +{ > + FILE *f =3D qemu_log_trylock(); This can fail, and if it does... > + if (log_depth =3D=3D 1 && message_with_timestamp) { > + g_autofree const char *timestr =3D NULL; > + g_autoptr(GDateTime) dt =3D g_date_time_new_now_utc(); > + timestr =3D g_date_time_format_iso8601(dt); > + fprintf(f, "%s ", timestr); ... we crash here. > + } > + return f; > } >=20=20 > void qemu_log_unlock(FILE *logfile) > { > + assert(log_depth); > + log_depth--; > if (logfile) { > fflush(logfile); > qemu_funlockfile(logfile); > @@ -145,28 +171,9 @@ void qemu_log_unlock(FILE *logfile) >=20=20 > void qemu_log(const char *fmt, ...) > { > - FILE *f; > - g_autofree const char *timestr =3D NULL; > - > - /* > - * Prepare the timestamp *outside* the logging > - * lock so it better reflects when the message > - * was emitted if we are delayed acquiring the > - * mutex > - */ > - if (message_with_timestamp) { > - g_autoptr(GDateTime) dt =3D g_date_time_new_now_utc(); > - timestr =3D g_date_time_format_iso8601(dt); > - } > - > - f =3D qemu_log_trylock(); > + FILE *f =3D qemu_log_trylock_context(); > if (f) { > va_list ap; > - > - if (timestr) { > - fprintf(f, "%s ", timestr); > - } > - > va_start(ap, fmt); > vfprintf(f, fmt, ap); > va_end(ap);