linux-kernel - Re: [RFC v2 PATCH 0/4] futex: Add support task local hash maps.

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Message-ID: <20241028175008.lgdGYYtp@linutronix.de>
Date: Mon, 28 Oct 2024 18:50:08 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-kernel@...r.kernel.org
Cc: André Almeida <andrealmeid@...lia.com>,
	Darren Hart <dvhart@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Valentin Schneider <vschneid@...hat.com>,
	Waiman Long <longman@...hat.com>
Subject: Re: [RFC v2 PATCH 0/4] futex: Add support task local hash maps.

On 2024-10-28 13:13:54 [+0100], To linux-kernel@...r.kernel.org wrote:
> from time to time and I haven't seen it using the hashmap. Need to do
> more testing.

booted gnome and did a few things:
- Total allocations: 632
- tasks which never used their allocated futex hash: 2
  gpg-agent and systemd-journal
- Tasks which did not terminate within the measurement: 85
  This includes gpg-agent and systemd-journal
- Top5 users of the private hash:
  - firefox-esr-3786 used 215985
  - gnome-software-2343 used 121296
  - chromium-3369 used 65796
  - chromium-3209 used 34639
  - Isolated used 34211

This looks like we could attach the private futex hashmap directly on
fork instead of delaying it to the first usage.

Side note: If someone is waiting for a thread to exit via pthread_join()
then glibc uses here futex() with op 0x109. I would have expected a
private flag. 

Sebastian