From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Eric Dumazet <edumazet@google.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
David Laight <David.Laight@aculab.com>,
Ingo Molnar <mingo@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
paulmck <paulmck@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
Arjun Roy <arjunroy@google.com>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs()
Date: Tue, 13 Apr 2021 14:00:12 -0400 (EDT) [thread overview]
Message-ID: <1347243835.72576.1618336812739.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <CANn89iLXE6V2gpbJeE6KVU+YiNkmYZKjpRxKv8b69k1ECsyE9g@mail.gmail.com>
----- On Apr 13, 2021, at 1:33 PM, Eric Dumazet edumazet@google.com wrote:
> On Tue, Apr 13, 2021 at 7:20 PM Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>>
>> ----- On Apr 13, 2021, at 1:07 PM, Eric Dumazet edumazet@google.com wrote:
>>
>> > On Tue, Apr 13, 2021 at 7:01 PM Eric Dumazet <edumazet@google.com> wrote:
>> >>
>> >> On Tue, Apr 13, 2021 at 6:57 PM Eric Dumazet <edumazet@google.com> wrote:
>> >> >
>> >> > On Tue, Apr 13, 2021 at 6:54 PM Mathieu Desnoyers
>> >> > <mathieu.desnoyers@efficios.com> wrote:
>> >> > >
>> >> > > ----- On Apr 13, 2021, at 12:22 PM, Eric Dumazet eric.dumazet@gmail.com wrote:
>> >> > >
>> >> > > > From: Eric Dumazet <edumazet@google.com>
>> >> > > >
>> >> > > > Commit ec9c82e03a74 ("rseq: uapi: Declare rseq_cs field as union,
>> >> > > > update includes") added regressions for our servers.
>> >> > > >
>> >> > > > Using copy_from_user() and clear_user() for 64bit values
>> >> > > > is suboptimal.
>> >> > > >
>> >> > > > We can use faster put_user() and get_user().
>> >> > > >
>> >> > > > 32bit arches can be changed to use the ptr32 field,
>> >> > > > since the padding field must always be zero.
>> >> > > >
>> >> > > > v2: added ideas from Peter and Mathieu about making this
>> >> > > > generic, since my initial patch was only dealing with
>> >> > > > 64bit arches.
>> >> > >
>> >> > > Ah, now I remember the reason why reading and clearing the entire 64-bit
>> >> > > is important: it's because we don't want to allow user-space processes to
>> >> > > use this change in behavior to figure out whether they are running on a
>> >> > > 32-bit or in a 32-bit compat mode on a 64-bit kernel.
>> >> > >
>> >> > > So although I'm fine with making 64-bit kernels faster, we'll want to keep
>> >> > > updating the entire 64-bit ptr field on 32-bit kernels as well.
>> >> > >
>> >> > > Thanks,
>> >> > >
>> >> >
>> >> > So... back to V1 then ?
>> >>
>> >> Or add more stuff as in :
>> >
>> > diff against v2, WDYT ?
>>
>> I like this approach slightly better, because it moves the preprocessor ifdefs
>> into
>> rseq_get_rseq_cs and clear_rseq_cs, while keeping the same behavior for a 32-bit
>> process running on native 32-bit kernel and as compat task on a 64-bit kernel.
>>
>> That being said, I don't expect anyone to care much about performance of 32-bit
>> kernels, so we could use copy_from_user() on 32-bit kernels to remove
>> special-cases
>> in 32-bit specific code. This would eliminate the 32-bit specific "padding"
>> read, and
>> let the TASK_SIZE comparison handle the check for both 32-bit and 64-bit
>> kernels.
>>
>> As for clear_user(), I wonder whether we could simply keep using it, but change
>> the
>> clear_user() macro to figure out that it can use a faster 8-byte put_user ? I
>> find it
>> odd that performance optimizations which would be relevant elsewhere creep into
>> the
>> rseq code.
>
>
> clear_user() is a maze of arch-dependent macros/functions/assembly
>
> I guess the same could be said from copy_in_user(), but apparently we removed
> special-casing, like in commit a41e0d754240fe8ca9c4f2070bf67e3b0228aa22
>
> Definitely it seems odd having to carefully choose between multiple methods.
As long as the ifdefs are localized within clearly identified wrappers in the
rseq code I don't mind doing the special-casing there.
The point which remains is that I don't think we want to optimize for speed
on 32-bit architectures when it adds special-casing and complexity to the 32-bit
build. I suspect there is less and less testing performed on 32-bit architectures
nowadays, and it's good that as much code as possible is shared between 32-bit and
64-bit builds to share the test coverage.
Thanks,
Mathieu
>
>
>>
>> Thanks,
>>
>> Mathieu
>>
>> >
>> > diff --git a/kernel/rseq.c b/kernel/rseq.c
>> > index
>> > f2eee3f7f5d330688c81cb2e57d47ca6b843873e..537b1f684efa11069990018ffa3642c209993011
>> > 100644
>> > --- a/kernel/rseq.c
>> > +++ b/kernel/rseq.c
>> > @@ -136,6 +136,10 @@ static int rseq_get_cs_ptr(struct rseq_cs __user **uptrp,
>> > {
>> > u32 ptr;
>> >
>> > + if (get_user(ptr, &rseq->rseq_cs.ptr.padding))
>> > + return -EFAULT;
>> > + if (ptr)
>> > + return -EINVAL;
>> > if (get_user(ptr, &rseq->rseq_cs.ptr.ptr32))
>> > return -EFAULT;
>> > *uptrp = (struct rseq_cs __user *)ptr;
>> > @@ -150,8 +154,9 @@ static int rseq_get_rseq_cs(struct task_struct *t,
>> > struct rseq_cs *rseq_cs)
>> > u32 sig;
>> > int ret;
>> >
>> > - if (rseq_get_cs_ptr(&urseq_cs, t->rseq))
>> > - return -EFAULT;
>> > + ret = rseq_get_cs_ptr(&urseq_cs, t->rseq);
>> > + if (ret)
>> > + return ret;
>> > if (!urseq_cs) {
>> > memset(rseq_cs, 0, sizeof(*rseq_cs));
>> > return 0;
>> > @@ -237,7 +242,8 @@ static int clear_rseq_cs(struct task_struct *t)
>> > #ifdef CONFIG_64BIT
>> > return put_user(0UL, &t->rseq->rseq_cs.ptr64);
>> > #else
>> > - return put_user(0UL, &t->rseq->rseq_cs.ptr.ptr32);
>> > + return put_user(0UL, &t->rseq->rseq_cs.ptr.ptr32) |
>> > + put_user(0UL, &t->rseq->rseq_cs.ptr.padding);
>> > #endif
>> > }
>>
>> --
>> Mathieu Desnoyers
>> EfficiOS Inc.
> > http://www.efficios.com
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
next prev parent reply other threads:[~2021-04-13 18:00 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-13 16:22 [PATCH v2 0/3] rseq: minor optimizations Eric Dumazet
2021-04-13 16:22 ` [PATCH v2 1/3] rseq: optimize rseq_update_cpu_id() Eric Dumazet
2021-04-13 16:22 ` [PATCH v2 2/3] rseq: remove redundant access_ok() Eric Dumazet
2021-04-13 16:22 ` [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs() Eric Dumazet
2021-04-13 16:54 ` Mathieu Desnoyers
2021-04-13 16:57 ` Eric Dumazet
2021-04-13 17:01 ` Eric Dumazet
2021-04-13 17:07 ` Eric Dumazet
2021-04-13 17:20 ` Mathieu Desnoyers
2021-04-13 17:33 ` Eric Dumazet
2021-04-13 18:00 ` Mathieu Desnoyers [this message]
2021-04-13 18:22 ` Eric Dumazet
2021-04-13 18:35 ` Arjun Roy
2021-04-13 21:19 ` David Laight
2021-04-13 22:03 ` Arjun Roy
2021-04-14 7:55 ` David Laight
2021-04-14 16:00 ` Eric Dumazet
2021-04-14 16:08 ` David Laight
2021-04-14 16:10 ` Eric Dumazet
2021-04-14 17:15 ` Arjun Roy
2021-04-14 17:35 ` Eric Dumazet
2021-04-14 20:15 ` Arjun Roy
2021-04-14 20:25 ` Eric Dumazet
2021-04-14 20:35 ` Arjun Roy
2021-04-13 19:13 ` Mathieu Desnoyers
2021-04-13 17:06 ` Mathieu Desnoyers
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