From: Peter Hurley <peter@hurleysoftware.com>
To: lkp@lists.01.org
Subject: Re: increased vmap_area_lock contentions on "n_tty: Move buffers into n_tty_data"
Date: Thu, 26 Sep 2013 13:22:08 -0400 [thread overview]
Message-ID: <52446D40.9040504@hurleysoftware.com> (raw)
In-Reply-To: <20130926153208.GN18242@two.firstfloor.org>
[-- Attachment #1: Type: text/plain, Size: 1471 bytes --]
On 09/26/2013 11:32 AM, Andi Kleen wrote:
> On Thu, Sep 26, 2013 at 07:52:23AM -0400, Peter Hurley wrote:
>> On 09/25/2013 11:20 PM, Andi Kleen wrote:
>>> Lin Ming <minggr@gmail.com> writes:
>>>>
>>>> Would you like below patch?
>>>
>>> The loop body keeps rather complex state. It could easily
>>> get confused by parallel RCU changes.
>>>
>>> So if the list changes in parallel you may suddenly
>>> report very bogus values, as the va_start - prev_end
>>> computation may be bogus.
>>>
>>> Perhaps it's ok (may report bogus gaps), but it seems a bit risky.
>>
>> I don't understand how the computed gap would be bogus; there
>> _was_ a list state in which that particular gap existed. The fact
>
> It could change any time as you don't have an atomic view
> of vm_end / vm_start. It is valid to change the fields
> with the lock held.
va_start and va_end are constant for the lifetime of their vmap_area
(if it's accessible by traversing the vmap_area_list), so it is
not possible for an rcu-based list traversal to see different
values of these individual fields than the spin-locked version.
In addition, for the rcu-based traversal to have arrived at any given
vmap_area requires that the previous vmap_area was its adjacent
lower range at the instant in time when the list cursor was advanced;
again, this is no different than if the spin-locked version had
happened to begin at that same instant.
Regards,
Peter Hurley
WARNING: multiple messages have this Message-ID (diff)
From: Peter Hurley <peter@hurleysoftware.com>
To: Andi Kleen <andi@firstfloor.org>
Cc: Lin Ming <minggr@gmail.com>,
Fengguang Wu <fengguang.wu@intel.com>,
Greg KH <gregkh@linuxfoundation.org>,
LKML <linux-kernel@vger.kernel.org>,
lkp@01.org, Tejun Heo <tj@kernel.org>
Subject: Re: increased vmap_area_lock contentions on "n_tty: Move buffers into n_tty_data"
Date: Thu, 26 Sep 2013 13:22:08 -0400 [thread overview]
Message-ID: <52446D40.9040504@hurleysoftware.com> (raw)
In-Reply-To: <20130926153208.GN18242@two.firstfloor.org>
On 09/26/2013 11:32 AM, Andi Kleen wrote:
> On Thu, Sep 26, 2013 at 07:52:23AM -0400, Peter Hurley wrote:
>> On 09/25/2013 11:20 PM, Andi Kleen wrote:
>>> Lin Ming <minggr@gmail.com> writes:
>>>>
>>>> Would you like below patch?
>>>
>>> The loop body keeps rather complex state. It could easily
>>> get confused by parallel RCU changes.
>>>
>>> So if the list changes in parallel you may suddenly
>>> report very bogus values, as the va_start - prev_end
>>> computation may be bogus.
>>>
>>> Perhaps it's ok (may report bogus gaps), but it seems a bit risky.
>>
>> I don't understand how the computed gap would be bogus; there
>> _was_ a list state in which that particular gap existed. The fact
>
> It could change any time as you don't have an atomic view
> of vm_end / vm_start. It is valid to change the fields
> with the lock held.
va_start and va_end are constant for the lifetime of their vmap_area
(if it's accessible by traversing the vmap_area_list), so it is
not possible for an rcu-based list traversal to see different
values of these individual fields than the spin-locked version.
In addition, for the rcu-based traversal to have arrived at any given
vmap_area requires that the previous vmap_area was its adjacent
lower range at the instant in time when the list cursor was advanced;
again, this is no different than if the spin-locked version had
happened to begin at that same instant.
Regards,
Peter Hurley
next prev parent reply other threads:[~2013-09-26 17:22 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-13 0:51 increased vmap_area_lock contentions on "n_tty: Move buffers into n_tty_data" Fengguang Wu
2013-09-13 0:51 ` Fengguang Wu
2013-09-13 1:09 ` Fengguang Wu
2013-09-13 1:09 ` Fengguang Wu
2013-09-17 15:34 ` Peter Hurley
2013-09-17 15:34 ` Peter Hurley
2013-09-17 23:22 ` Fengguang Wu
2013-09-17 23:22 ` Fengguang Wu
2013-09-18 0:22 ` Peter Hurley
2013-09-18 0:22 ` Peter Hurley
2013-09-25 9:04 ` Lin Ming
2013-09-25 9:04 ` Lin Ming
2013-09-25 11:30 ` Peter Hurley
2013-09-25 11:30 ` Peter Hurley
2013-09-25 14:53 ` Lin Ming
2013-09-25 14:53 ` Lin Ming
2013-09-25 16:02 ` Lin Ming
2013-09-25 16:02 ` Lin Ming
2013-09-26 3:20 ` Andi Kleen
2013-09-26 3:20 ` Andi Kleen
2013-09-26 11:52 ` Peter Hurley
2013-09-26 11:52 ` Peter Hurley
2013-09-26 15:32 ` Andi Kleen
2013-09-26 15:32 ` Andi Kleen
2013-09-26 17:22 ` Peter Hurley [this message]
2013-09-26 17:22 ` Peter Hurley
2013-09-26 7:33 ` Andrew Morton
2013-09-26 7:33 ` Andrew Morton
2013-09-26 11:31 ` Peter Hurley
2013-09-26 11:31 ` Peter Hurley
2013-09-26 15:04 ` Greg KH
2013-09-26 15:04 ` Greg KH
2013-09-26 17:35 ` Peter Hurley
2013-09-26 17:35 ` Peter Hurley
2013-09-26 18:05 ` Andrew Morton
2013-09-26 18:05 ` Andrew Morton
2013-09-26 21:42 ` Peter Hurley
2013-09-26 21:42 ` Peter Hurley
2013-09-26 21:58 ` Andrew Morton
2013-09-26 21:58 ` Andrew Morton
2013-09-26 22:21 ` Peter Hurley
2013-09-26 22:21 ` Peter Hurley
2013-09-18 0:49 ` Peter Hurley
2013-09-18 0:49 ` Peter Hurley
2013-09-13 3:17 ` Greg KH
2013-09-13 3:17 ` Greg KH
2013-09-13 3:38 ` Fengguang Wu
2013-09-13 3:38 ` Fengguang Wu
2013-09-13 3:44 ` Greg KH
2013-09-13 3:44 ` Greg KH
2013-09-13 9:55 ` Peter Hurley
2013-09-13 9:55 ` Peter Hurley
2013-09-13 12:34 ` Greg KH
2013-09-13 12:34 ` Greg KH
2013-09-17 2:42 ` Peter Hurley
2013-09-17 2:42 ` Peter Hurley
2013-09-17 2:56 ` Fengguang Wu
2013-09-17 2:56 ` Fengguang Wu
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=52446D40.9040504@hurleysoftware.com \
--to=peter@hurleysoftware.com \
--cc=lkp@lists.01.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.