linux-kernel - Re: [PATCH 00/25] hrtimer: Provide softirq context hrtimers

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Date:   Thu, 31 Aug 2017 14:36:03 +0200 (CEST)
From:   Anna-Maria Gleixner <anna-maria@...utronix.de>
To:     Christoph Hellwig <hch@....de>
cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, keescook@...omium.org,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 00/25] hrtimer: Provide softirq context hrtimers

I'm sorry for the typo in the email address!

    Anna-Maria

On Thu, 31 Aug 2017, Anna-Maria Gleixner wrote:

> There are quite some places in the kernel which use a combination of
> hrtimers and tasklets to make use of the precise expiry of hrtimers, which
> schedule a tasklet to bring the actual function into softirq context.
> 
> This was introduced when the previous hrtimer softirq code was
> removed. That code was implemented by expiring the timer in hard irq
> context and then deferring the execution of the callback into softirq
> context. That caused a lot of pointless shuffling between the rbtree and a
> linked list.
> 
> In recent discussions it turned out that more potential users of hrtimers
> in softirq context might come up. Aside of that the RT patches need this
> functionality as well to defer hrtimers into softirq context if their
> callbacks are not interrupt safe on RT.
> 
> This series implements a new approach by adding SOFT_* clock ids and
> instead of doing the list shuffle, timers started with these clock ids are
> put into separate soft expiry hrtimer queues. These queues are evaluated
> only when the hardirq context detects that the first expiring timer in the
> softirq queues has expired. That makes the overhead in the hardirq context
> minimal.
> 
> The series reworks the code to reuse as much as possible from the existing
> facilities for the new softirq hrtimers and integrates them with all
> flavours of hrtimers (HIGH_RES=y/n - NOHZ=y/n).
> 
> To achieve this quite some of the conditionals in the existing code are
> removed for the price of adding some pointless data and state tracking to
> the HIGH_RES=n case. That's minimal, but well worth it as it increases the
> readability and maintainability of the code.
> 
> The first part of the series implements the new functionality and the
> second part converts the hrtimer/tasklet users to make use of it and
> removes struct hrtimer_tasklet and the surrounding helper functions.
> 
> This series is available from git as well:
> 
>     git://git.kernel.org/pub/scm/linux/kernel/git/bigeasy/linux-hrtimer.git soft_hrtimer
>     https://git.kernel.org/bigeasy/linux-hrtimer/h/soft_hrtimer
> 
> 
> Thanks,
> 
> 	Anna-Maria
> 
> ---
>  drivers/net/usb/cdc_ncm.c             |   37 +-
>  drivers/net/wireless/mac80211_hwsim.c |   44 +-
>  drivers/usb/gadget/function/f_ncm.c   |   28 -
>  include/linux/hrtimer.h               |   76 ++---
>  include/linux/interrupt.h             |   25 -
>  include/linux/usb/cdc_ncm.h           |    2 
>  include/net/xfrm.h                    |    2 
>  kernel/softirq.c                      |   51 ---
>  kernel/time/hrtimer.c                 |  513 +++++++++++++++++++++-------------
>  net/can/bcm.c                         |  150 +++------
>  net/xfrm/xfrm_state.c                 |   29 +
>  sound/drivers/dummy.c                 |   16 -
>  12 files changed, 484 insertions(+), 489 deletions(-)
> 
>