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Date:   Wed, 8 Sep 2021 16:12:16 +0000
From:   Wei Liu <wei.liu@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Wei Liu <wei.liu@kernel.org>,
        Naresh Kamboju <naresh.kamboju@linaro.org>,
        Mathias Nyman <mathias.nyman@intel.com>,
        Johannes Berg <johannes@sipsolutions.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Brendan Higgins <brendanhiggins@google.com>,
        Ariel Elior <aelior@marvell.com>,
        GR-everest-linux-l2@marvell.com,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        open list <linux-kernel@vger.kernel.org>,
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        Arnd Bergmann <arnd@arndb.de>,
        "David S. Miller" <davem@davemloft.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Eric Dumazet <edumazet@google.com>
Subject: Re: ipv4/tcp.c:4234:1: error: the frame size of 1152 bytes is larger
 than 1024 bytes [-Werror=frame-larger-than=]
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On Wed, Sep 08, 2021 at 08:59:36AM -0700, Linus Torvalds wrote:
> On Wed, Sep 8, 2021 at 3:03 AM Wei Liu <wei.liu@kernel.org> wrote:
> >
> > Thanks for the heads-up. I found one instance of this bad practice in
> > hv_apic.c. Presumably that's the one you were referring to.
> 
> Yeah, that must have been the one I saw.
> 
> > However calling into the allocator from that IPI path seems very heavy
> > weight. I will discuss with fellow engineers on how to fix it properly.
> 
> In other places, the options have been fairly straightforward:
> 
>  (a) avoid the allocation entirely.
> 
> I think the main reason hyperv does it is because it wants to clear
> the "current cpu" from the cpumask for the ALL_BUT_SELF case, and if
> you can just instead keep track of that "all but self" bit separately
> and pass it down the call chain, you may not need that allocation at
> all.
[..]
> 
> That said, if you are already just iterating over the mask, doing (a)
> may be trivial. No allocation at all is even better than a percpu one.
> 

Yep. I just wrote two patches for this approach.

Wei.